Graphite film and graphite composite film

ABSTRACT

An object of the present invention is to provide a graphite film, and a graphite composite film both having an excellent thermal diffusivity which can sufficiently manage heat dissipation of electronic instruments, precision instruments and the like, along with an excellent flex resistance which can withstand application to bent portions. Means for Resolution of the present invention is a graphite film exhibiting the number of reciprocal foldings being 10,000 times or more as measured using a rectangular strip test piece having a width of 15 mm until the test piece breaks in a MIT folding endurance test under conditions of: a curvature radius R of the bending clamp being 2 mm; a left-and-right bending angle being 135°; a bending rate being 90 times/min; and a load being 0.98 N.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a continuation of U.S. application Ser. No. 12/600,389, filedApr. 19, 2010, which is the U.S. national phase of InternationalApplication No. PCT/JP2008/058910, filed May 15, 2008, which claims thebenefit of Japanese patent Application No. 2007-132120, filed May 17,2007.

TECHNICAL FIELD

The present invention relates to a graphite film for use as a heatspreader material and a radiator film of electronic instruments,precision instruments and the like, and particularly relates to agraphite film that is excellent in flex resistance and thermaldiffusivity.

BACKGROUND ART

Cooling issues in semiconductor elements mounted in variouselectronic/electric instruments such as computers, and in otherheat-generating components have been emphasized. As a cooling method ofsuch components which should be cooled, a method including attaching afan to an instrument housing in which the component is mounted, therebycooling the instrument housing; a method including attaching a heatconductor such as a heat pipe, a heat spreader, a heat sink or a fin tothe component to be cooled, thereby cooling is executed by transferringoutward the heat from the element; or the like may be generallyemployed. Examples of a heat-conducting material attached to a componentwhich should be cooled include aluminum plates, copper plates, and thelike. In such a case, a heat-generating component is attached to a partof the aluminum or copper plate, or to the heat pipe, whereby furtherheat dissipation of other portion of the plate is allowed outside usinga fin or a fan.

Meanwhile, downsizing of each instrument into which a heat-generatingcomponent such as a semiconductor element is mounted has been performedin recent years, and the amount of heat generation of the member thereoftends to be larger. However, due to downsizing of the housing, the spaceinto which a component such as a fin or a heat sink, a fan and the likecan be inserted has been limited.

Thus, graphite films that are excellent in thermal diffusivity have beenregarded as a promising heat conductor in recent years. A graphite filmhas a layered structure formed with carbon, and is a material having avery high thermal conductivity in the face thereof, having a density ofthe film being approximately 1 to 2 g/cm³, thus being light, and havinga high electric conductivity. In addition since the sheet can be thinnedin the thickness and is flexible, it is expected as a heat conductormaterial or a heat spreader material for use in narrow places, or placesin which affixing around through gaps is required.

For the present, commonly available graphite films may be exemplified bygraphite films manufactured by a polymer thermal degradation method oran expansion method.

The polymer thermal degradation method is, as disclosed in PatentDocuments 1 and 2, a method in which a polymer film of polyoxadiazole,polyimide, polyphenylenevinylene, polybenzoimidazole, polybenzooxazole,polythiazole, polyamide or the like is subjected to a heat treatment inan inert atmosphere of argon, helium or the like, or under reducedpressure, whereby a graphite film is obtained.

-   Patent Document 1: JP-A No. Sho 61-275116-   Patent Document 2: JP-A No. Hei 2-103478    On the other hand, the graphite is obtained by immersing powdery or    squamous natural black lead, which is used as a source material, in    an acid and thereafter widening the space between the graphite    layers by heating, in an expansion method. Additionally,    high-pressure press processing is carried out with a caking    additive, thereby yielding a filmy graphite.

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

Structures of current electronic instruments become increasingly complexwhile advances of downsizing of such instruments have made; therefore,the way how to allow the heat to escape efficiently with a small spacesignificantly matters. There are several attempts to use such a graphitefilm being folded, which is characterized by having flexibility, in aspace-saving part. For example, an idea of dissipating the heat from aheat generation part in flip-open mobile phones or laptop computerstoward the liquid crystal side through the bent portion has beenprojected.

However, a fracture is often generated in a graphite film at the bentportion thereof since it is materially fragile, particularly when it isfolded with a small curvature radius or with a large bending angle. Inparticular, although a graphite film obtained by an expansion method hasflexibility, it has small graphite crystallite accompanied by inferiorcrystallinity since it is produced from a powdery or squamous naturalblack lead as a source material. Therefore, such a graphite film hasless thermal diffusivity and less film strength, and is thus morefragile as compared with a graphite obtained by a polymer thermaldegradation method. Additionally, although a graphite film obtained by apolymer degradation method (Patent Documents 1 to 2) has flex resistanceand thermal diffusivity to some extent, it is not satisfactory as aheat-dissipating material for recent electronic instruments accompaniedby advanced downsizing and complexity.

In this regard, Patent Document 3 proposes a method of manufacturing agraphite sheet that exhibits physical properties similar to those ofsingle-crystal graphite, and has a high quality, superior flexibilityand toughness, with the thermal diffusivity being excellent. Thismanufacturing method is fundamentally characterized by including a firstheat treatment step carried out using a polyimide film as a sourcematerial in an inert gas in the upper limit temperature range of 1,000°C. to 1,600° C.; and a second heat treatment step further carried out inthe upper limit temperature range of 2,500° C. to 3,100° C., and furthercontrolling the heat treatment conditions such as a temperatureelevation rate, a constant temperature and the like to manufacture agraphite sheet having appropriate properties, which may be subjectedfurther to a rolling treatment, whereby the flexibility is achieved.Furthermore, a method of manufacturing a graphite film is disclosed suchas a method as referred to in claim 7 of Patent Document 3 in whichprovided that the graphite sheet has a density falling within the rangeof 0.3 to 0.7 g/cc or the graphite sheet has a film thickness fallingwithin the range of two to ten times the film thickness of the sourcematerial film, a graphite sheet that is excellent in flexibility andtoughness can be obtained when a rolling treatment is carried out as apost-treatment, or as referred to in claim 8 of Patent Document 3 inwhich provided that the graphite sheet obtained by a rolling treatmenthas a density falling within the range of 0.7 to 1.5 g/cc, the resultinggraphite sheet has excellent flexibility and toughness.

-   Patent Document 3: JP-A No. 2000-178016    However, even in the case of the graphite film produced by the    manufacturing method disclosed in Patent Document 3, the film is    likely to be broken when it is bent with a small curvature radius,    or with a large bending angle, and sufficient flex resistance for    introducing into recent downsized electronic instruments may not be    exhibited.

In addition, “flex resistance” and “thermal diffusivity” arecontradictory properties. As disclosed in Patent Document 3, the flexresistance can be improved by producing in the film a space to make thegraphite layer movable in bending in the graphitization process;however, this space inhibits migration of heat, whereby the thermaldiffusivity is reduced. The flex resistance is sought also by thegraphite film produced by the manufacturing method of Patent Document 3;therefore, it has inferior heat diffusibility, and cannot deal with theincrease in the amount of heat generation in current electronicinstruments.

Moreover, a technique of providing a reinforcing material on one face orboth two faces of a graphite film to enhance the mechanical strength wasproposed. For example, Patent Document 4 discloses an expanded blacklead laminate sheet in which a plastic film is overlaid on both twofaces of an expanded black lead sheet, and at least a part of theplastic film is welded to be joined to the expanded black lead sheet onthese polymerized faces.

Furthermore, Patent Document 5 discloses a heat-conducting sheet,connected by thermal fusion bonding of a plastic tape on at least oneface of a graphite film. In addition, as in Patent Document 6, aheat-dissipating sheet is disclosed in which a polymer material layerhaving lower rigidity than that of the graphite film is provided,whereby deformation without breaking of the graphite film is enabledeven though a stress such as bending is caused in the graphite film,since the adjacent polymer material layer absorbs the stress by way ofthe shear to reduce the stress of the graphite film.

-   Patent Document 4: JP-A No. Hei 6-134917-   Patent Document 5: JP-A No. Hei 11-58591-   Patent Document 6: JP-A No. 2003-168882    However, even though deficient strength is compensated with a    supporting member such as a plastic tape, the graphite film as    disclosed in Patent Documents 1 to 6 cannot exhibit flex resistance    sufficient for use in current electronic instruments, when bent with    a small curvature radius or with a large bending angle, since only    the graphite film is broken although the support itself may not be    broken.

Accordingly, several attempts to improve flex resistance of a graphitefilm have been made; however, properties sufficient for use in mountingin recent electronic instruments have not yet achieved, and thusdevelopment of a graphite film having high thermal diffusivity and beingexcellent in flex resistance has been demanded.

Means for Solving the Problems

A first aspect of the present invention relates to a graphite filmcharacterized by exhibiting the number of reciprocal foldings being10,000 times or more as measured using a rectangular strip test piecehaving a width of 15 mm until the test piece breaks in a MIT foldingendurance test under conditions of: a curvature radius R of the bendingclamp being 2 mm; a left-and-right bending angle being 135 degrees; abending rate being 90 times/min; and a load being 0.98 N, the graphitefilm being obtained by subjecting a source material film consisting of apolymer film and/or a carbonized polymer film to a heat treatment at atemperature of 2,000° C. or higher, and the graphite film beingcharacterized by, on an SEM image of the surface at a magnification of400×, having the area of a white region accounting for not less than 1%and not greater than 8.5% of the image defined by binarization to blackand white with a threshold of 160 and further thinning of the whiteregion of the binarized image.

Furthermore, it is preferred that the graphite film be characterized byexhibiting the number of reciprocal foldings being 10,000 times or moreas measured using a rectangular strip test piece having a width of 15 mmuntil the test piece breaks in a MIT folding endurance test underconditions of: a curvature radius R of the bending clamp being 1 mm; aleft-and-right bending angle being 135 degrees; a bending rate being 90times/min; and a load being 0.98 N, and the graphite film has acoefficient of thermal diffusivity in a planer direction of not lessthan 8.0×10⁻⁴ m²/s.

Moreover, a second aspect of the present invention relates to a graphitecomposite film characterized in that a plastic film is formed on a partof any one the graphite film described above via an adhesive material.

Effects of the Invention

A graphite film and a graphite composite film that are excellent in flexresistance and thermal diffusivity are provided.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a cross-sectional schematic view of a foamed graphite film;

FIG. 2 shows a cross-sectional schematic view of a compressed graphitefilm;

FIG. 3 shows an SEM photograph of a foamed graphite film (large extentof foaming, small domain);

FIG. 4 shows an SEM photograph of a foamed graphite film (small extentof foaming, large domain);

FIG. 5 shows an SEM photograph of a compressed graphite film (largeextent of foaming, small domain);

FIG. 6 shows an SEM photograph of a compressed graphite film (smallextent of foaming, large domain);

FIG. 7 shows an explanatory view of an edge effect;

FIG. 8 shows an explanatory view of the region separated by a boundaryline;

FIG. 9 shows an explanatory view of an image processing;

FIG. 10 shows a view illustrating a polyimide film and a wedge-shapedsheet;

FIG. 11 shows a perspective view illustrating a wedge-shaped sheet;

FIG. 12 shows a view illustrating a composite with a PET tape;

FIG. 13 shows a view illustrating a composite with a flexible printwiring plate;

FIG. 14 shows a view illustrating a method of evaluating a coolingperformance after seam folding;

FIG. 15 shows a view illustrating a method of holding a source materialfilm by a vessel A;

FIG. 16 shows a view illustrating a method of holding by a vessel A anda vessel B, and a method of electrification;

FIG. 17 shows an SEM photograph of the surface after image processing inExample 1;

FIG. 18 shows an SEM photograph of the surface after image processing inExample 2;

FIG. 19 shows an SEM photograph of the surface after image processing inExample 3;

FIG. 20 shows an SEM photograph of the surface after image processing inExample 4;

FIG. 21 shows an SEM photograph of the surface after image processing inExample 5;

FIG. 22 shows an SEM photograph of the surface after image processing inExample 6;

FIG. 23 shows an SEM photograph of the surface after image processing inComparative Example 1;

FIG. 24 shows an SEM photograph of the surface after image processing inComparative Example 2;

FIG. 25 shows an SEM photograph of the surface after image processing inComparative Example 3;

FIG. 26 shows an SEM photograph of the surface after image processing inComparative Example 4;

FIG. 27 shows an SEM photograph of the surface after image processing inComparative Example 5;

FIG. 28 shows an SEM photograph of the surface after image processing inReference Example 1; and

FIG. 29 shows an SEM photograph of the surface after image processing inReference Example 2.

11 domain size 12 extent of foaming 13 cross-sectional schematic view ofa foamed graphite film 21 size of region surrounded by a boundary line22 boundary line 23 cross-sectional schematic view of a compressedgraphite film 71 smooth portion 72 protruding portion 73 edge portion 74secondary electron 75 specimen 76 diffusive region 81 region separatedby a boundary line 82 boundary line 91 SEM image 92 image afterbinarization 93 image after thinning 1 polyimide film 2 wedge-shapedsheet 3 width of wedge-shaped sheet 4 sodium light 5 interference fringe121 PET 122 adhesive material 123 graphite film 124 runover by 1 mm 125side view 126 top view 131 PET 132 adhesive material 133 polyimide film134 graphite film 135 copper foil 141 silicone rubber sheet 142 heater143 seam folded portion 144 graphite film 151 vessel A holding thesource material film 152 smooth electrically accessible flat plate forholding the source material film 161 vessel B 162 carbon particlesfilled between vessel A and vessel B 163 carbon particles filled aroundouter periphery of vessel B

BEST MODE FOR CARRYING OUT THE INVENTION

A first aspect of the present invention is a graphite film characterizedby exhibiting the number of reciprocal foldings being 10,000 times ormore as measured using a rectangular strip test piece having a width of15 mm until the test piece breaks in a MIT folding endurance test underconditions of: a curvature radius R of the bending clamp being 2 mm; aleft-and-right bending angle being 135 degrees; a bending rate being 90times/min; and a load being 0.98 N.

Furthermore, a second aspect of the present invention is a graphite filmcharacterized by exhibiting the number of reciprocal foldings being10,000 times or more as measured using a rectangular strip test piecehaving a width of 15 mm until the test piece breaks in a MIT foldingendurance test under conditions of: a curvature radius R of the bendingclamp being 1 mm; a left-and-right bending angle being 135 degrees; abending rate being 90 times/min; and a load being 0.98 N.

<Graphite Film>

As a countermeasure for dealing with increase in heat generation densityin recent electronic instruments, graphite films that are significantlyfavorable in thermal diffusivity have drawn attention. For the present,there are commonly available graphite films manufactured by a polymerthermal degradation method or an expansion method.

<Manufacturing Method of the Graphite Film of the Present Invention>

According to the method for manufacturing the graphite film of thepresent invention, a source material film consisting of a polymer filmand/or a carbonized polymer film is subjected to a heat treatment at atemperature of 2,000° C. or higher, thereby obtaining a graphite film.It is known that in general, a graphite film obtained by a polymerthermal degradation method in which a source material film is subjectedto a heat treatment in an inert atmosphere such as argon or helium, orunder reduced pressure is excellent in flex resistance and thermaldiffusivity.

Whereas, although a graphite film obtained by an expansion method hasflexibility, it is inferior in crystallinity having small graphitecrystallite, since it is produced by compacting powdery or squamousnatural black lead. Therefore, thus obtained ones are inferior inthermal diffusivity as compared with the graphite obtained by thepolymer thermal degradation method, and many have less film strength andare fragile.

<Flex Resistance of Graphite Film>

The number of foldings with a MIT folding endurance test of the graphitefilm of the present invention (with R being 2 mm, and a left-and-rightbending angle being 135°) is 10,000 times or more, preferably 50,000times or more, and still more preferably 100,000 times or more. When thenumber of the foldings is 10,000 times or more, excellent flexresistance is achieved, and is less likely to be broken even though itis used in bent portion. Specifically, even though it is used in hingesof mobile phones and bent portions of downsized electronic instruments,use without deteriorating the function is enabled. In addition, due tothe excellent flex resistance, handlability in attaching to electronicinstruments can be also improved. To the contrary, when the number offoldings is less than 10,000 times, the film is likely to be brokenduring use in the bent portion due to inferior flex resistance.Additionally, handlability during use may be also wrong. Particularly,when the bending angle is large, and when the bending radius is small,the film is easily deteriorated.

In addition, the number of foldings of the graphite film of the presentinvention with a MIT folding endurance test (with R being 1 mm, and aleft-and-right bending angle being 135°) is 10,000 times or more,preferably 50,000 times or more, and still more preferably 100,000 timesor more. When the number of the foldings is 10,000 times or more,excellent flex resistance is achieved, and is less likely to be brokeneven though it is used in bent portion. Specifically, even though it isused in hinges of mobile phones and bent portions of downsizedelectronic instruments, use without deteriorating the function isenabled. In addition, due to the excellent flex resistance, handlabilityin attaching to electronic instruments can be also improved. To thecontrary, when the number of foldings is less than 10,000 times, thefilm is likely to be broken during use in the bent portion due toinferior flex resistance. Additionally, handlability during use may bealso wrong. Particularly, when the bending angle is large, and when thebending radius is small, the film is easily deteriorated.

<Bending Radius and Bending Angle in MIT Folding Endurance Test>

The MIT folding endurance test of a graphite film may be measured usinga MIT flex resistant fatigue testing machine model D manufactured byTOYO SEIKI Co., Ltd., or the like. In the measurement, the bendingradius R and the bending angle can be selected, with the option of Rbeing 2 mm, 1 mm and the like. Generally, the testing conditions will bemore severe as the bending radius R is smaller, and as the bending angleis larger. Particularly, in electronic instruments having smaller spacesuch as mobile phones, gaming devices, liquid crystal televisions, PDPsand the like, it is very important to achieve superior foldability withsmaller bending radius and larger bending angle since, space-savingdesigns of the instruments are enabled. It should be noted that detailsof the MIT test process are described in EXAMPLES below.

<Mechanism for Graphite Film Production Excellent in Flex Resistance>

The graphite film that is excellent in the flex resistance is obtainedby elevating the temperature of a polymer film such as a polyimide filmto 2,600° C. or higher. In a final stage of graphitization (2,600° C. orhigher), the graphite layer is raised by an internal gas generated suchas N₂, whereby the film is foamed (irregularity resulting from thefoaming being observed on the surface as shown in FIG. 1). Thus foamedgraphite film is subjected to pressing or rolling, whereby a graphitefilm that is excellent in the flex resistance is produced. The foamedgraphite film is excellent in the flex resistance on the ground that thestrain of the graphite layer applied in folding can be relieved due tothe presence of the air layer between the layers of the graphite.

<Extent of Foaming and Flex Resistance>

However, all thus foamed graphite films are not necessarily excellent inthe flex resistance. Depending on the graphitization conditions, thesize of foaming in the thickness direction of the graphite film(hereinafter, referred to as extent of foaming) varies, and the extentof foaming is the primary factor that decides whether the flexresistance is superior or inferior. When the extent of foaming is toosmall, the amount of the air layer that exists between the graphitelayers is insufficient, and thus a film that is hard and inferior in theflex resistance is yielded since the strain of the graphite layer cannotbe relieved in bending. In addition, a graphite film that is inferior inthe flex resistance is yielded also in the case in which the extent offoaming is too large, as the graphite layer is likely to be detachedfrom the surface in folding since the graphite layer is broken forfoaming. In FIG. 1, a part indicating the extent of foaming is shown inthe schematic view of a cross section of a graphite film.

<Domain Size and Flex Resistance>

Moreover, the size of each one irregularity in the planer direction(domain size) determined on the graphite surface in the foaming statealso varies depending on the conditions of graphitization, and thisdomain size is also one factor that that decides whether the flexresistance is superior or inferior. When the domain size is too large, ahard film is yielded as the flex resistance is inferior and hence itcannot stand the folding. In addition, when the domain size is toosmall, each one domain is likely to be detached in folding, therebyresulting in a graphite film having inferior flex resistance.

As described in the foregoing, it is necessary to optimize thegraphitization conditions to produce an appropriate foaming state forthe purpose of providing a graphite having a high flex resistance.

<Boundary Line on the Surface of Graphite Film after Compression>

As described above, irregularities can be observed on the surface of thegraphite film after foaming as shown in FIG. 1. When a compressionprocessing such as pressing or rolling is carried out, theseirregularities collapse, and the graphite layers overlap on theboundaries of the irregularities, whereby the boundary line as in theschematic view shown in FIG. 2 is generated.

<Relationship Between Foaming State of Graphite Film and Boundary Lineon the Surface>

When the extent of foaming is large, or when the foaming domain size issmall, many boundary lines are generated on the surface after thecompression processing. To the contrary, when the extent of foaming issmall, or when the foaming domain size is large, boundary lines are lesslikely to be generated on the surface after the compression processing.Therefore, by determining the amount of the boundary lines on thesurface of the graphite film after the compression processing, thefoaming state of the graphite film can be identified, and thus the flexresistance can be presumed as to whether it is superior or inferior.

By way of examples, FIGS. 3 and 4 show SEM photographs of graphite filmshaving different foaming states. FIG. 3 shows the extent of foamingbeing larger, while the domain size being smaller. When the graphitefilms shown in FIGS. 3 and 4 are compressed, the irregularities of thefoaming are collapsed, and the boundary lines are generated on thesurface as shown in FIGS. 5 and 6. FIG. 5 shows a graphite film having alarger extent of foaming, and a smaller domain size compressed,revealing a large amount of generation of boundary lines, while FIG. 6shows a graphite film having a smaller extent of foaming, and a largerdomain size, revealing a small amount of generation of boundary lines.

<Observation of Boundary Line on Graphite Surface by SEM Inspection>

The boundary line of a graphite film on the surface can be observed bySEM inspection. When an SEM inspection is carried out on a graphitesurface, the part of the boundary line looks bright as shown in FIGS. 5and 6.

The principle for such observation looking bright at the part of theboundary line is referred to as an edge effect, in which a largequantity of secondary electrons are emitted from the corners of thespecimen surface or the projection tip, and thus such parts becomeenormously brighter compared to other parts, in a secondary electronimage as shown in FIG. 7.

<Details of SEM Inspection of Graphite Film Surface>

The observation of the surface of a graphite film was carried out usinga scanning electron microscope (SEM) (product name: manufactured byHitachi, model S-4500) at an accelerating voltage of 5 kV. Several kindsof graphite films were cut into pieces of 5 mm×5 mm, and fixed on analminum stage having a diameter of 15 mm with an electrically conductivetape. The stage is regulated and set to give a height of 36 mm. Theaccelerating voltage was regulated to be 5 eV, and observation with ahigh magnification mode at 400× was carried out. The working distancewas regulated to be 8 mm, and then the brightness, the contrast and thefocus were adjusted. Thus, photographing was carried out such that thewrinkle of the graphite film surface can be observed. The image wascaptured at 640×480.

<Size of Region Separated by Boundary Line of the Graphite Film of thePresent Invention>

The mean area of the region separated by the boundary line of thegraphite film of the present invention is not less than 25 μm² and notgreater than 10,000 μm², preferably not less than 100 μm² and notgreater than 6,400 μm², and more preferably not less than 200 μm² andnot greater than 3,600 μm². When the area is less than 25 μm²,detachment of the graphite layer is likely to be generated in folding,and hence the flex resistance is inferior. Further, when the area isgreater than 10,000 μm², the flex resistance becomes inferior sincestrain of the graphite layer cannot be relieved in bending, and a hardfilm is provided. To the contrary, when the area is not less than 25 μm²and not greater than 10,000 μm², a graphite film that is excellent inthe flex resistance can be provided. It should be noted that the regionseparated by a boundary line is deemed as being separated provided that75% or more of the periphery of the region is separated as shown in FIG.8.

<Image Processing of SEM Inspection Image on Graphite Film Surface>

Image processing of the image obtained by SEM inspection of the graphitefilm surface was carried out using a general-purpose image processingsoft (product name: NANO HUNTER NS2K-PRO/LT) available from NANO SystemCorporation. With respect to details of the image processing, the SEMimage was first incorporated into the image processing program, and thedensity measurement was carried out. The maximum value (maximum: 255)and the minimum value (0) measured with the density measurement wereconfirmed, and the threshold of the binarization was determinedaccording to the following formula.Threshold=(maximum value−minimum value)×0.62  (1)

Next, the SEM image was binarized with the threshold determinedaccording to the above formula. The binarization is a processing inwhich a region revealing brighter than a certain threshold is whitened,while a region revealing darker than a certain threshold is blackened.

Subsequently, the binarized image was thinned. In the thinningprocessing, the whitened portion in the binarized image is converted ina line width of 1.

The area of the white region of the image derived by the processing asdescribed above was measured. As an example, an SEM photograph and theimage following carrying out the image processing are shown in FIG. 9.

<Surface State of the Graphite Film of the Present Invention>

The area of the white region of the graphite film of the presentinvention following carrying out the image processing after the SEMinspection at a magnification of 400× accounts for not less than 1.0%and not greater than 8.5%, preferably not less than 1.2 and not greaterthan 6.5%, and more preferably not less than 1.4% and not greater than4.2%. When the area of the white region accounts for less than 1.0%, theboundary line of the graphite film surface appears in a small amount,suggesting that the graphite film is inferior in the flex resistance.Further, when the area of the white region accounts for greater than8.5%, the boundary line appears in a too large amount, meaning that thegraphite film is inferior in the flex resistance as the graphite layeris likely to be fallen off from the graphite surface due to foldingoperations. To the contrary, when the area of the white region accountsfor not less than 1.0% and not greater than 8.5%, each one regionseparated by the boundary line is large to some extent, meaning that thegraphite film is excellent in the flex resistance.

<Method for Manufacturing the Graphite Film of the Present Invention>

In the graphite film according to the present invention, appropriatespaces that make the graphite layers movable upon bending are producedinside the film by a graphitization process. Specifically, the sourcematerial film is subjected to a heat treatment at 2,400° C. or higher,and foaming is permitted between the graphite layers utilizinggeneration of a nitrogen gas remaining in the source material.

As described above, it is important to optimize the foaming state whichis one factor that decides the flex resistance and thermal diffusivityof a graphite film. By observing the boundary line of the graphite filmsurface as described above, the flex resistance and thermal diffusivityof a graphite film can be predicted.

Factors that control the foaming state of a graphite film (extent offoaming, domain size) include:

(1) the heating method in graphitization;

(2) the maximum temperature in graphitization;

(3) the number of laminated pieces of the source material film;

(4) the pressure applied to the film in graphitization;

(5) the molecular orientation of the source material film;

and the like, and the details are explained below. For the optimizationof the foaming state, it is important that these factors are wellbalanced, respectively, and the above five conditions should be alteredad libitum. When the above five factors are combined with favorablebalance, thereby enabling the extent of foaming, domain size and thelike to be produced, a graphite film that is very excellent in the flexresistance and thermal diffusivity can be obtained.

<Carbonization Step and Graphitization Step>

The graphitization step of the present invention may be either carriedout taking the polymer film carbonized by the carbonization step outfrom the furnace for the carbonization once and thereafter displacingthe same into a furnace for graphitization, or may be carried out in thesame furnace by carrying out the carbonization step, and thegraphitization step successively.

<Heating Method for Graphitization>

Conventionally, atmospheric heating carried out under reduced pressureor in an inert gas has been known as a graphitization step. In theatmospheric heating method, a voltage is applied to a heater made fromblack lead to allow for heating. The heat generated from the heater istransferred to the specimen by way of radiation, and convection of theinert gas. As the inert gas used in the atmospheric heating method,argon and helium are suited.

In addition, as a method for producing a high quality graphite film, anelectric heating method has been known. In the electric heating method,a voltage is applied to the specimen to be the subject of heating, or avessel per se in which the specimen is packed, thereby carrying outdirect heating.

<Atmospheric Heating Method>

In common heat treatment methods in an atmosphere and under reducedpressure conventionally employed, heating is carried out with heattransfer of the atmosphere gas and/or a radiation heat from the heater,or heat transfer form the portion that is in contact with the heater.Thus, uneven heating of a film proceeds by the heat transfer basicallyfrom the film surface inwardly; therefore, partial fluctuation may occurin growth of the graphite layer; adverse effects due to the decomposedgas generated during the graphitization may be exerted; or partialdefects are likely to be generated during rearrangement of the crystals.Accordingly, formation of a graphite film having nonuniform domain sizeand extent of foaming has been obliged. Therefore, a graphite film thatis inferior in the flex resistance and thermal diffusivity has beenoften obtained.

One cause for yielding such a graphite film having nonuniform domainsize and extent of foaming is spacing between the graphite layers thatoccurs when the element other than carbon included in the startingsource material is gasified and escaped. In the case in which thisoperation is carried out under a reduced-pressure atmosphere, the gas isgenerated rapidly from the film due to the reduced pressure, wherebygraphite layers are exfoliated to cause detachment of the graphite,which may lead to deterioration of the appearance. In addition, suchevents can break bonds of the graphite in the planer direction, wherebylowering of the flex resistance and thermal conductivity may be caused.

<Electric Heating Method>

On the other hand, the electric heating method is preferably executed byan “electric heating” system in which a polymer film and/or a carbonizedpolymer film is brought into direct contact with and held in anelectrically accessible vessel (direct electrification vessel), andgraphitization is allowed while electrification by applying to thevessel a counter current voltage and/or a direct current voltage. Inthis system, concomitant with permitting heat generation of the vesselitself, heating through electrification is carried out by applying avoltage to the source material film as a consequence; therefore, heatgeneration of the source material film itself is contributory. In otherwords, to carry out the graphitization step by an electrification systemis advantageous since the film is heated by two means, i.e., direct heattransfer from the heat-generating vessel, and self heat generation ofthe film; therefore, the film is heated evenly both inside and on thesurface, and also heated from around the film evenly enough, whereby anappropriate foaming state is achieved as homogeneous graphitizationproceeds both on the surface and the inside, as a consequence.

Moreover, since the graphite layers evenly grow in the face of thegraphite film obtained via a graphitization step by an electric heatingsystem, the resultant graphite film is more likely to have favorabledensity and coefficient of thermal diffusivity, as well as flatnesswithout flaw, wrinkle and dint even though the film is subjected to arolling treatment or a compression treatment, and also the film is moreexcellent in the flex resistance and thermal diffusivity thanconventional ones.

<Method for Treatment of Electric Heating>

As the graphitization step by electric heating according to the presentinvention, for example, a method in which a source material film is heldin a black lead vessel, and a voltage is applied to this black leadvessel itself to permit electrification; a method in which a sourcematerial film is held in a black lead vessel, which black lead vessel iscovered by (filled with) carbon powders around the outer periphery, anda voltage is applied to this black lead vessel itself to permitelectrification via the carbon powders; a method in which a sourcematerial film covered by carbon powders is held in a black lead vessel(held in a state in which carbon powders are filled between the blacklead vessel and the source material film), and a voltage is applied tothis black lead vessel itself to permit electrification; a method inwhich a source material film covered by carbon powders is held in ablack lead vessel (held in a state in which carbon powders are filledbetween the black lead vessel and the source material film), which blacklead vessel is further covered by carbon powders (in a state in whichcarbon powders are filled around the outer periphery of the black leadvessel), and a voltage is applied to this black lead vessel itself topermit electrification via the carbon powders; and the like may beconceived.

<Maximum temperature of Graphitization>

When the maximum temperature of graphitization is low, the degree ofprogress of the graphitization, the extent of foaming, and the degree ofgrowth of the domain all become small. In the method for manufacturingthe graphite film of the present invention, the heat treatmenttemperature of 2,000° C. or higher is necessary at lowest and finally2,800° C. or higher, more preferably 2,900° C. or higher and finallypreferably 3,000° C. or higher. By thus setting a heat treatmenttemperature, the graphite layers grow in the planer direction to allowthe growth with a large domain size, whereby a graphite film that isexcellent in the flex resistance and thermal diffusivity is obtained. Tothe contrary, when the maximum temperature of graphitization is lowerthan 2,800° C., graphitization may not proceed enough in a part of thefilm. When the graphitization does not proceed enough, the graphite filmwill be hard, and inferior in the flex resistance and thermaldiffusivity since insufficient extent of foaming is attained.

As the heat treatment temperature is higher, transformation into ahigher quality graphite is enabled, the temperature is preferably as lowas possible in view of the economical efficiency. In the method formanufacturing the graphite film of the present invention, the heattreatment temperature is no higher than 3,500° C. at highest, morepreferably no higher than 3,200° C., and still more preferably no higherthan 3,100° C. In industrial furnaces which are generally available forthe present, the maximum temperature which can be employed in a heattreatment is limited to 3,000° C.

<Number of Laminated Pieces of Source Material Film>

The film is heated in an even manner both inside and on the surface, andalso heated from around the film evenly enough according to the electricheating as described above; therefore, it is advantageous in thatappropriate foaming state is achieved as a result, due to thehomogeneous graphitization both on the surface and the inside. However,when the source material film is heated too evenly, a graphite film withvery small extent of foaming, and having too large domain may beobtained. In such a graphite film, since graphite layers are very highlyoriented in the planer direction, a graphite film that is very inferiorin the flex resistance is likely to be obtained due to failure incorresponding to the strain of the graphite layer generated in folding,although a high heat conductivity may be achieved.

In addition, as described above, the source material film and/or theblack lead vessel are covered by carbon particles during the heattreatment according to the electric heating as described above, and thusfilm foaming in graphitization may be suppressed during the heattreatment due to impurities such as metals invaded from the outside, theblack lead vessel and the carbon particles, as well as gasses from theoutside. Since the film in which foaming is suppressed has graphitelayers very highly oriented in the planer direction, a graphite filmthat is very poor in the flex resistance is likely to be obtained due tofailure in corresponding to the strain of the graphite layer generatedin folding, although a high heat conductivity may be achieved.

In order to solve the problems as described in the foregoing,graphitization is carried out in the graphitization step according tothe present invention, in a state of the source material films beinglaminated. The number of the laminated pieces is not less than 10,preferably not less than 30, and more preferably not less than 50.

By laminating the source material films, heterogeneity in progress ofthe graphitization may occur to some extent as compared with the case inwhich a single piece of the source material film alone is used, sincethe proportion of the source material film occupying the vessel iselevated. Owing to this heterogeneity, a graphite film having anadequate extent of foaming and an adequate domain size can be obtained.Such a graphite film is very excellent in the flex resistance andthermal diffusivity.

Moreover, the source material film and/or the black lead vessel iscovered by the carbon particles during the heat treatment as describedabove in the electric heating, and thus erosion and deterioration may becaused during the heat treatment due to impurities such as metalsinvaded from the outside, the black lead vessel and the carbonparticles, as well as gasses from the outside. When a source materialfilm laminate in which a plurality of pieces of the source materialfilms are laminated is used as the source material film as in thepresent invention, the source material film is in a closed contactstate. Therefore, it becomes hardly affected by impurities invaded fromthe outside, whereby a graphite film that is excellent in the flexresistance and thermal diffusivity can be produced in large quantities.

Furthermore, when a source material film laminate in which a pluralityof pieces of the source material films are laminated is used as thesource material film as in the present invention, the source materialfilm is in a closed contact state, thereby allowing the gas to be lesslikely to be escaped; therefore, retardation of timing of generation ofthe gas is enabled until reaching to a temperature region to result indevelopment of the graphite layers. As a consequence, since foaming iscarried out in the state after reaching to a planer state withoutdeteriorating the graphite layer, a graphite film having an adequateextent of foaming and an adequate domain size is obtained. To thecontrary, when a single film without lamination of the source materialfilms as conventional products is used as a starting source material, astate in which the gas is likely to be escaped from both faces of thefilm; therefore, the gas is likely to be escaped prior to formation ofthe graphite layer, whereby widening of the space between graphitelayers may be difficult.

When a source material film laminate in which a plurality of pieces ofthe source material films are laminated is used as the source materialfilm as in the present invention, the source material film serves as abuffering material in escaping the gas. Therefore, the force applied tothe source material film upon deformation during heating can be reduced,and thus the bonds of the graphite layers are not broken. Consequently,excellent flex resistance and thermal diffusivity can be realized.

To the contrary, when a simplicial film in which source material filmsare not laminated is used as the source material film as in conventionalcases, it is necessary to sandwich the source material film with agraphite plate, a spacer such as a graphite film, a carbon plate or acarbon film. In such a case, the flex resistance and thermal diffusivitymay be deteriorated since the generated gas may be prevented fromescaping due to pressure by the spacer, and the bonds of the graphitelayers are broken.

By carrying out graphitization with the laminated source material filmin such a manner, a graphite film that is excellent in the flexresistance and thermal diffusivity can be obtained. However, the numberof the laminated pieces of the source material film illustrated hereinis merely one example, and is not limited thereto, which may bedetermined ad libitum depending on the heating method of graphitizationand the maximum temperature of graphitization described above, and thepressure applied to the film upon graphitization and the orientation ofthe source material film described later.

<Pressure Applied to Film in Graphitization>

When the film is significantly compressed in the graphitization, thecompression leads to physical orientation of the graphite layers in theplaner direction, whereby a graphite film having a small extent offoaming and a large domain size is likely to be obtained. To thecontrary, when the film is poorly compressed in graphitization, anonuniform graphite film having a large extent of foaming and a smalldomain size is likely to be obtained.

The pressure applied to the source material film in the thicknessdirection in the present invention may be not less than 5.0 g/cm², andmore preferably not greater than 100 g/cm². In graphitization of thesource material film, a process in which the size of the source materialfilm is expanded and/or shrunk is included. When graphitization iscarried out under the pressure being less than 5.0 g/cm², expansionand/or shrinkage of the nonuniform film accompanying to graphitizationof the source material film may occur, and thus homogeneousgraphitization is not achieved in the film face, thereby resulting in agraphite film having a large extent of foaming, nonuniform domain sizeand inferior flex resistance.

To the contrary, when the pressure is not less than 5.0 g/cm² and notgreater than 100 g/cm², when the polyimide film and/or carbonizedpolyimide film as described later is particularly used as a sourcematerial film, a graphite film that is excellent in the heatconductivity could be obtained. This is presumed to result fromacceleration of development of the graphite crystal structure in theplaner direction of the film accompanying to graphitization in thegraphitization step, as the graphitization was carried out whileapplying a pressure in the thickness direction of the source materialfilm.

In addition, when the pressure is higher than 100 g/cm², development ofthe graphite crystal structure in the planer direction may beexcessively accelerated, whereby a graphite having very small extent offoaming, and a very large domain size is obtained. The aforementionedpressure is calculated with respect to the area of the graphite filmobtained following the heat treatment.

In the graphitization step, the way for achieving application of apressure to the source material film in the film thickness direction mayinvolve: the own weight of a jig used in holding the film; a pressurefrom a lid in the case in which the lid is used for the vessel forholding the film; expansion of the vessel around the film due toheating; and a pressure resulting from the expansion of the jig used inholding the film, but is not limited thereto.

Moreover, with respect to an appropriate pressure applied to the film ingraphitization, the heating method of graphitization and the maximumtemperature of graphitization described above, and the number of thelaminated pieces of the source material film and the orientation of thesource material film described later may be determined ad libitum, butnot limited thereto. For example, when the number of the laminatedpieces is large, it is necessary to increase the applied pressure so asto lessen the extent of foaming, since the heating is likely to beuneven, and the extent of foaming tends to be great as described above.To the contrary, when the number of the laminated pieces is small, theapplied pressure may be small.

<Source material Film>

The source material film which can be used in the present invention is apolymer film or a carbonized polymer film.

<Polymer Film>

The polymer film which can be used in the present invention is notparticularly limited, but is exemplified by polyimide (PI), polyamide(PA), polyoxadiazole (POD), polybenzooxazole (PBO), polybenzobisoxazole(PBBO), polythiazole (PT), polybenzothiazole (PBT), polybenzobisthiazole(PBBT), polyparaphenylenevinylene (PPV), polybenzoimidazole (PBI) andpolybenzobisimidazole (PBBI), and the polymer film is preferably a heatresistant aromatic polymer film including at least one selected fromthese, since the flex resistance and thermal diffusivity of the graphitefinally obtained can be large. These films may be manufactured by awell-known manufacturing method. Of these, polyimide is preferred sincethe film having a variety of structures and properties can be obtainedby selecting the source material monomer variously. In addition,polyimide films are likely to lead to a graphite having superiorcrystallinity and being excellent in the flex resistance and thermaldiffusivity, since carbonization and graphitization of the film proceedmore easily than polymer films produced using other organic material asa source material.

<Carbonized Polymer Film>

The carbonized polymer film used in the present invention is preferablyobtained by subjecting a polymer film used as a starting substance to apreheating treatment under reduced pressure or in an inert gas. It isdesired that this preheating is carried out at a temperature of usually1,000° C., for example, when the temperature is elevated at a rate of10° C./min, the temperature is kept in the temperature region of 1,000°C. for 30 min. More specifically, carbonization temperature forcarbonizing the polymer film may be 600° C. or higher and lower than2,000° C. Thus, a carbonized polymer film used as an example of thesource material film according to the present invention is preferably acarbonized polymer film obtained by subjecting a polymer film to a heattreatment at a temperature of 600 to 1,800° C. The temperature of theheat treatment temperature is preferably 1,000° C. or higher, morepreferably 1,100° C. or higher, still more preferably 1,200° C. orhigher, and particularly preferably 1,400° C. or higher. The temperatureof carbonization is preferably lower than 2,000° C. on the ground thatgraphitization is carried out by the electric heating as describedlater. The temperature of carbonization is preferably 600° C. or higheron the ground that source material films are likely to be hardly adheredeach other during the heat treatment in the case of graphitizationcarried out following lamination, and that displacement of the positionof the source material film during the graphitization step due to thedecomposed gas and deformation can be prevented, and as a consequence,wrinkles and fractures of the resulting graphite film can be prevented.In other words, shrinkage of the film occurs in the thickness directionand the planer direction in the carbonization step, while shrinkage ofthe film occurs in the thickness direction and expansion occurs in theplaner direction in the graphitization step; therefore, when a polymerfilm is used as the source material film, shrinkage of the film in theplaner direction is suppressed when a pressure is applied in thethickness direction, whereby wrinkles and fractures of the film mayoccur.

However, even though a pressure is applied in the thickness direction,expansion in the planer direction of the film is promoted by using acarbonized polymer film as the source material film, whereby qualitiesof the graphite film are likely to be excellent. Additionally, to use acarbonized polymer film as the source material film is preferred, inrespect of deformation of the film being able to be suppressed moresignificantly in comparison with the case of a polymer film, and alsoprevention of displacement of the position of the film due todeformation being enabled. Furthermore, the source material film and/orthe black lead vessel are/is covered by carbon particles described laterduring the heat treatment in the electric heating. When a carbonizedpolymer film is used as the source material film, the source materialfilm becomes compact, has greater resistance to corrosion, and becomesless likely to be eroded and/or deteriorated during the heat treatmentdue to impurities such as metals invaded from the outside, the blacklead vessel and the carbon particles, as well as gasses from theoutside, whereby a graphite film that is excellent in the flexresistance and thermal diffusivity and has lower variation of qualitiesin the face (particularly, center and end of the film) can be producedin large quantities.

Additionally, to use the carbonized polymer film as the source materialfilm is preferred, since the electric current flows both in the surfacelayer and inside, in the graphitization step by electric heating toresult in heat generation concurrently proceeds both in the surfacelayer and inside, whereby homogeneous graphitization occurs. Thus, agraphite film that is excellent in the flex resistance and thermaldiffusivity can be obtained.

<Polyimide Film>

In a polyimide film, carbonization and graphitization of the film ismore liable to proceed than a source material film including an otherorganic material as a source material; therefore, the electricconductivity of the film easily becomes uniform at low temperatures, andthe electric conductivity per se is also likely to be higher. As aresult, when graphitization is carried out by applying a voltage asdescribed later while holding the source material film directly in theelectrically accessible vessel, and permitting electrification byapplying a voltage to the vessel, the electric current flows in the filmportion uniformly as carbonization proceeds, whereby uniform heatgeneration is caused in the surface and the inside. Thus a graphitehaving high thermal diffusivity can be obtained not only in the case inwhich the thickness is small, but also in the case of thick films.Moreover, since the completed graphite has superior crystallinity, andis also excellent in the heat resistance, breakage can be obviated eventhough local heat is generated through concentration of the electricfield, whereby a high quality graphite that is excellent in the flexresistance and thermal diffusivity can be obtained.

<Polyimide Film and Birefringence>

The birefringence Δn in connection with the in-plane orientation of themolecules in the polymer film of the present invention is not less than0.08, preferably not less than 0.10, more preferably 0.12, and mostpreferably not less than 0.14, in any direction in the film face. Whenthe birefringence is not less than 0.08, carbonization andgraphitization of the film become more likely to proceed. As a result,more favorable crystal orientation of the graphite is attained, and theflex resistance and thermal diffusivity can be markedly improved. Inaddition, as the birefringence becomes greater, carbonization andgraphitization of the film can proceed more efficiently, whereby theelectric conductivity of the film described later is more likely to beelevated. As a result, in the step of graphitization by applying avoltage while holding the source material film directly in theelectrically accessible vessel, and permitting electrification byapplying a voltage to the vessel, the electric current flows in the filmportion uniformly as carbonization proceeds, and the amount of theelectric current that flows in the film increases as carbonizationproceed, whereby homogeneous graphitization is more likely to proceeddue to uniform heat generation that occurs in the surface and theinside. In addition, since the electric conductivity is elevated evenlyin the film face, partial electric field concentration does not occurand thus local heat generation is not caused, whereby homogeneousgraphitization proceeds as a result in the surface and the inside.Moreover, since the carbonization and the graphitization proceed at lowtemperatures, the electric conductivity of the film during the heattreatment at even low temperatures is elevated, whereby homogeneousgraphitization is more likely to proceed as uniform heat generationoccurs in the surface and the inside. Further, crystallinity becomesmore superior and the heat resistance is improved as the birefringenceis higher; therefore, a high quality graphite film is obtained withoutbreakage even though the electric field is concentrated to generate alocal heat.

The reason for facilitation of graphitization as the birefringenceincreases is not clear; however, a graphite film that is excellent inthe flex resistance and thermal diffusivity can be obtained with morehighly orientated polyimide films among the polyimide films, sincerearrangement of molecules required for graphitization can be minimizedin the polyimide film having a greater birefringence and superior in themolecular orientation.

<Birefringence>

The term “birefringence” referred to herein means a difference between arefractive index in any in any in-plane direction of a film and arefractive index in the thickness direction. The birefringence Δnx inany in-plane direction X of a film is derived by the following formula(mathematical formula 1).Birefringence Δnx=(refractive index Nx in the in-plane directionX)−(refractive index Nz in the thickness direction)  [Mathematicalformula 1]Birefringence Δnx=(Refractive index in the in-plane X directionNx)−(Refractive index in the thickness direction Nz)  (1)

FIGS. 10 and 11 illustrate a specific method for measuringbirefringence. Referring to a plan view of FIG. 11, a wedge-shaped sheet2 is cut out as a measurement specimen from a film 1. The wedge-shapedsheet 2 has a long trapezoidal shape with an oblique line, and one baseangle thereof is a right angle. The wedge-shaped sheet 2 is cut out suchthat the bottom of the trapezoid is parallel to the X direction. FIG. 11is a perspective view of the measurement specimen 2 thus cut out. Sodiumlight 4 is applied at right angles to a cutout cross-sectioncorresponding to the bottom of the trapezoidal specimen 2, and a cutoutcross-section corresponding to the oblique line of the trapezoidalspecimen 2 is observed with a polarization microscope. Thereby,interference fringes 5 are observed. The birefringence Δnx in thein-plane direction X is represented by the following formula(mathematical formula 2):Δnx=n*λ/d  [Mathematical formula 2]

wherein, λ is the wavelength of sodium D ray, i.e., 589 nm, and d is thewidth 3 of the specimen corresponding to the height of the trapezoid ofthe specimen 2.

Note that the term “in an in-plane direction X of a film” means that,for example, the X direction is any one of in-plane directions of 0°,45°, 90°, and 135° on the basis of the direction of flow of materialsduring the formation of the film. The measurement place and themeasurement time of the specimen are preferably as in the following. Forexample, when a specimen is cut away from a roll-shaped source materialfilm (width: 514 mm), sampling is conducted at six sites in the widthdirection with an interval of 10 cm, and the birefringence is measuredfor each site. The average is determined as the birefringence.

<Thermal Properties, Mechanical Properties, Physical Properties andChemical Properties of Polyimide Film>

Furthermore, the polyimide film used in the present invention, which isa source material for the graphite, may have a mean coefficient oflinear expansion of less than 2.5×10⁻⁵/° C. in a range of 100° C. to200° C. When the coefficient of linear expansion is less than 2.5×10⁻⁵/°C., the graphitization proceeds smoothly with less elongation during theheat treatment, whereby a graphite that is not fragile and has excellentvarious properties can be obtained. By using such a polyimide film for asource material, transformation into a graphite starts from 2,400° C.and transformation into a graphite of sufficiently high crystallinitycan take place at 2,700° C. The coefficient of linear expansion is morepreferably no higher than 2.0×10⁻⁵/° C.

Note that the coefficient of linear expansion of the polymer film isobtained by the following method. Using a thermomechanical analyzer(TMA), a specimen is heated to 350° C. at a heating rate of 10° C./minand then air-cooled to room temperature. The specimen is heated again to350° C. at a heating rate of 10° C./min, and the mean coefficient oflinear expansion at 100° C. to 200° C. during the second heating ismeasured. Specifically, using a thermomechanical analyzer (TMA:SSC/5200H; TMA120C manufactured by Seiko Electronics Industry Co.,Ltd.), a film specimen with dimensions of 3 mm in width and 20 mm inlength is fixed on a predetermined jig, and measurement is performed inthe tensile mode under a load of 3 g in a nitrogen atmosphere.

Furthermore, the polyimide film used in the present invention preferablyhas an elastic modulus of 3.4 GPa or more from the standpoint thatgraphitization can be more easily performed. That is, when the elasticmodulus is 3.4 GPa or more, it is possible to avoid breakage of the filmresulting from shrinkage of the film during heat treatment. Thus, it ispossible to obtain graphite that is excellent in various properties.

Note that the elastic modulus of the film can be measured in accordancewith ASTM-D-882. The polyimide film more preferably has an elasticmodulus of 3.9 GPa or more, and still more preferably 4.9 GPa or more.When the elastic modulus of the film is less than 3.4 GPa, breakage anddeformation easily occur due to shrinkage of the film during heattreatment, and the resulting graphite tends to have inferiorcrystallinity, and deteriorated thermal diffusivity.

The coefficient of water absorption of the film was measured asdescribed below. For achieving absolute dryness, the film is dried at100° C. for 30 min to produce a 10 cm×square specimen having a thicknessof 25 μm. The weight of this specimen is measured and defined as A1. The10 cm×square specimen having a thickness of 25 μm was immersed indistilled water at 23° C. for 24 hrs, and water of the surface was wipedto remove, and the weight of the specimen was measured immediately afterthe removal. This weight is defined as A2. The coefficient of waterabsorption was determined according to the following formula.Coefficient of water absorption(%)=(A2−A1)/A1×100

<Production Method of Polyimide Film>

The polyimide film used in the present invention can be formed byflow-casting an organic solution of a polyamic acid which is a precursorof the polyimide, after mixing the solution with an imidizationaccelerator onto a support such as an endless belt or stainless steeldrum, followed by drying, baking and imidization. A known process can beused as the process for producing the polyamic acid used in the presentinvention. Usually, at least one aromatic dianhydride and at least onediamine are dissolved in substantially equimolar amounts in an organicsolvent. The resulting organic solution is stirred under controlledtemperature conditions until polymerization between the acid dianhydrideand the diamine is completed. Thereby, a polyamic acid is produced. Sucha polyamic acid solution is obtained usually at a concentration of 5% to35% by weight, and preferably 10% to 30% by weight. When theconcentration is in such a range, a proper molecular weight and solutionviscosity can be obtained.

As the polymerization method, any of the known methods can be used. Forexample, the following polymerization methods (1) to (5) are preferable.

(1) A method in which an aromatic diamine is dissolved in a polarorganic solvent, and a substantially equimolar amount of an aromatictetracarboxylic dianhydride is allowed to react therewith to performpolymerization.

(2) A method in which an aromatic tetracarboxylic dianhydride and a lessthan equimolar amount of an aromatic diamine compound with respectthereto are allowed to react with each other in a polar organic solventto obtain a pre-polymer having acid anhydride groups at both termini.Subsequently, polymerization is performed using an aromatic diaminecompound so as to be substantially equimolar with respect to thearomatic tetracarboxylic dianhydride.

In one preferable aspect of this embodiment, a method is provided inwhich a pre-polymer having the acid dianhydride at both termini issynthesized using diamine and acid dianhydride, and the pre-polymer isallowed to react with diamine that is different from the above diamine,thereby synthesizing polyamic acid.

(3) A method in which an aromatic tetracarboxylic dianhydride and anexcess molar amount of an aromatic diamine compound with respect theretoare allowed to react with each other in a polar organic solvent toobtain a pre-polymer having amino groups at both termini. Subsequently,after adding an additional aromatic diamine compound to the pre-polymer,polymerization is performed using an aromatic tetracarboxylicdianhydride such that the aromatic tetracarboxylic dianhydride and thearomatic diamine compound are substantially equimolar to each other.

(4) A method in which an aromatic tetracarboxylic dianhydride isdissolved and/or dispersed in a polar organic solvent, and thenpolymerization is performed using an aromatic diamine compound so as tobe substantially equimolar to the acid dianhydride.

(5) A method in which a mixture of substantially equimolar amounts of anaromatic tetracarboxylic dianhydride and an aromatic diamine are allowedto react with each other in a polar organic solvent to performpolymerization.

Among these, the polymerization methods shown in above (2) and (3) inwhich sequential control (sequence control; control of combination ofblock polymers, and linkage of the block polymer molecules) is used byway of a pre-polymer are preferable, because it is possible to easilyobtain a polyimide film having a high birefringence and a lowcoefficient of linear expansion by using such a method, and also becauseit becomes possible to easily obtain a graphite having highcrystallinity, and excellent density and thermal diffusivity byheat-treating this polyimide film. Furthermore, it is assumed that bythus controlling in a regular fashion, the overlap between aromaticrings increases, and graphitization is allowed to proceed easily even bylow-temperature heat treatment. In addition, when the content of theimide group is elevated for increasing the birefringence, carbonproportion in the resin is lowered, the yield of carbonization aftersubjecting to the black lead processing is lowered; however, thepolyimide film synthesized while conducting sequential control ispreferred because the birefringence can be increased without loweringthe carbon proportion in the resin. Since the carbon proportion iselevated, generation of decomposed gas can be inhibited, whereby agraphite film having favorable appearance is more likely to be obtained.Also, rearrangement of the aromatic ring can be suppressed, and thus agraphite film that is excellent in thermal diffusivity can be obtained.

In the present invention, examples of the acid dianhydride which can beused for the synthesis of the polyimide include pyromelliticdianhydride, 2,3,6,7-naphthalenetetracarboxylic dianhydride,3,3′,4,4′-biphenyltetracarboxylic dianhydride,1,2,5,6-naphthalenetetracarboxylic dianhydride,2,2′,3,3′-biphenyltetracarboxylic dianhydride, 3,3′,4,4′-benzophenonetetracarboxylic dianhydride, 2,2-bis(3,4-dicarboxyphenyl)propanedianhydride, 3,4,9,10-perylene tetracarboxylic dianhydride,bis(3,4-dicarboxyphenyl)propane dianhydride,1,1-bis(2,3-dicarboxyphenyl)ethane dianhydride,1,1-bis(3,4-dicarboxyphenyl)ethane dianhydride,bis(2,3-dicarboxyphenyl)methane dianhydride,bis(3,4-dicarboxyphenyl)ethane dianhydride, oxydiphthalic dianhydride,bis(3,4-dicarboxyphenyl)sulfone dianhydride, p-phenylenebis(trimelliticacid monoester acid anhydride), ethylene bis(trimellitic acid monoesteracid anhydride), bisphenol A bis(trimellitic acid monoester acidanhydride), and analogues thereof. These may be used alone or as amixture in combination of two or more at an appropriate ratio.

In the present invention, examples of the diamine which can be used forthe synthesis of the polyimide include 4,4′-oxydianiline,p-phenylenediamine, 4,4′-diaminodiphenyl propane, 4,4′-diaminodiphenylmethane, benzidine, 3,3′-dichlorobenzidine, 4,4′-diaminodiphenylsulfide, 3,3′-diaminodiphenyl sulfone, 4,4′-diaminodiphenyl sulfone,4,4′-diaminodiphenyl ether (4,4′-oxydianiline), 3,3′-diaminodiphenylether (3,3′-oxydianiline), 3,4′-diaminodiphenyl ether(3,4′-oxydianiline), 1,5-diaminonaphthalene,4,4′-diaminodiphenyldiethylsilane, 4,4′-diaminodiphenylsilane,4,4′-diaminodiphenylethylphosphine oxide, 4,4′-diaminodiphenylN-methylamine, 4,4′-diaminodiphenyl N-phenylamine, 1,4-diaminobenzene(p-phenylenediamine), 1,3-diaminobenzene, 1,2-diaminobenzene, andanalogues thereof. These may be used alone or as a mixture incombination of two or more at an appropriate ratio.

In particular, from the standpoint that the coefficient of linearexpansion can be decreased, the elastic modulus can be increased, andthe birefringence can be increased, use of an acid dianhydriderepresented by the following formula (1) as a source material ispreferable in the production of the polyimide film in the presentinvention.

In the formula, R₁ represents any one of divalent organic groupsrepresented by the following formula (2) to formula (14):

wherein, R₂, R₃, R₄, and R₅ may each represent any one selected from thegroup consisting of —CH₃, —Cl, —Br, —F, or —OCH₃.

By using the acid dianhydride described above, it is possible to obtaina polyimide film having a relatively low coefficient of waterabsorption, which is also preferable from the standpoint that foamingdue to moisture can be prevented in the graphitization process

In particular, use of any one of the benzene nucleus-containing organicgroups represented by the formula (2) to formula (14) 2 as R₁ in theacid dianhydride is preferable from the standpoint that the resultingpolyimide film has high molecular orientation, a low coefficient oflinear expansion, a high elastic modulus, a high birefringence, and alow coefficient of water absorption.

An acid dianhydride represented by the following formula (15) may beused as a source material in the synthesis of the polyimide in thepresent invention to further decrease the coefficient of linearexpansion, increase the elastic modulus, increase the birefringence, anddecrease the coefficient of water absorption.

In particular, with respect to a polyimide film produced using, as asource material, an acid dianhydride having a structure in which benzenerings are linearly bonded by two or more ester bonds, although foldedchains are involved, a highly linear conformation is easily formed as awhole, and the polyimide film has a relatively rigid property. As aresult, by using this source material, it is possible to decrease thecoefficient of linear expansion of the polyimide film, for example, to1.5×10⁻⁵/° C. or less. In addition, the elastic modulus can be increasedto 500 kgf/mm² (490 GPa) or more, and the coefficient of waterabsorption can be decreased to 1.5% or less.

The polyimide of the present invention is preferably synthesized usingp-phenylenediamine as a source material to further decrease thecoefficient of linear expansion, increase the elastic modulus, andincrease the birefringence.

Furthermore, in the present invention, the diamine most suitably usedfor the synthesis of the polyimide includes 4,4′-oxydianiline andp-phenylenediamine. The number of moles of one of these or both ispreferably 40 mole percent or more, more preferably 50 mole percent ormore, even more preferably 70 mole percent or more, and still morepreferably 80 mole percent or more relative to the total diaminecontent. Furthermore, p-phenylenediamine is included preferably in anamount of 10 mole percent or more, more preferably 20 mole percent ormore, even more preferably 30 mole percent or more, and still morepreferably 40 mole percent or more. If the contents of these diaminesare below the lower limits of these mole percent ranges, the resultingpolyimide film tends to have an increased coefficient of linearexpansion, a decreased elastic modulus, and a decreased birefringence.However, when the total diamine content is entirely composed ofp-phenylenediamine, it is difficult to obtain a thick polyimide filmwhich does not substantially foam. Therefore, use of 4,4′-oxydianilineis preferable. Additionally, the carbon proportion is lowered, therebyenabling to reduce the amount of generation of the decomposed gas. Thus,necessity for rearrangement of the aromatic ring is reduced, and agraphite that is excellent in the appearance and thermal diffusivity canbe obtained.

Most appropriate acid dianhydride used for synthesis of the polyimidefilm in the present invention is pyromellitic dianhydride and/orp-phenylenebis(trimellitic acid monoester acid dianhydride) representedby the formula (15), and it is preferred that the mole percent of these,alone or in terms of total moles of the two be not less than 40% bymole, further not less than 50% by mole, still further not less than 70%by mole, and yet further not less than 80% by mole based on the entireacid dianhydride. When the amount of the acid dianhydride used is lessthan 40% by mole, the resulting polyimide film will tend to have a largecoefficient of linear expansion, a small elastic modulus, and smallbirefringence.

Additionally, an additive such as carbon black or graphite may be addedto the polyimide film, polyamic acid, and the polyimide resin.

Preferred examples of the solvent for the synthesis of the polyamic acidinclude amide solvents, such as N,N-dimethylformamide,N,N-dimethylacetamide, and N-methyl-2-pyrrolidone, andN,N-dimethylformamide and N,N-dimethylacetamide are particularlypreferably used.

The polyimide may be produced using either a thermal cure method or achemical cure method. In the thermal cure method, a polyamic acid, whichis a precursor, is imidized by heating. In the chemical cure method, apolyamic acid is imidized using a dehydrating agent represented by anacid anhydride, such as acetic anhydride, and a tertiary amine, such aspicoline, quinoline, isoquinoline, or pyridine, as an imidizationaccelerator. Above all, a tertiary amine having a higher boiling point,such as isoquinoline, is more preferable because such a tertiary amineis not evaporated in the initial stage during the production process ofthe film and a catalytic effect is likely to be exhibited until thefinal step of drying. In particular, from the standpoints that theresulting film tends to have a low coefficient of linear expansion, ahigh elastic modulus, and a high birefringence and that rapidgraphitization is enabled at relatively low temperatures and a qualitygraphite can be obtained, chemical curing is preferable. Furthermore,combined use of the dehydrating agent and the imidization accelerator ispreferable because the resulting film can have a decreased coefficientof linear expansion, an increased elastic modulus, and an increasedbirefringence. Moreover, in the chemical cure method, since imidizationreaction proceeds more rapidly, the imidization reaction can becompleted for a short period of time in heat treatment. Thus, thechemical cure method has high productivity and is industriallyadvantageous.

In a specific process for producing a film using chemical curing, first,stoichiometric amounts or more of a dehydrating agent and an imidizationaccelerator composed of a catalyst are added to a polyamic acidsolution, the solution is flow-cast or applied onto a support, e.g., asupporting plate, an organic film, such as PET, a drum, or an endlessbelt, so as to be formed into a film, and an organic solvent isevaporated to obtain a self-supporting film. Subsequently, theself-supporting film is imidized while drying by heating to obtain apolyimide film. The heating temperature is preferably in a range of 150°C. to 550° C. Although the heating rate is not particularly limited,preferably, gradual heating is performed continuously or stepwise sothat the maximum temperature reaches the predetermined temperaturerange. The heating time depends on the thickness of the film and themaximum temperature. In general, the heating time is preferably 10seconds to 10 minutes after the maximum temperature is achieved.Moreover, it is preferable to include a step of making the film contactwith or fix/be held on the vessel, or drawing the film in order toprevent shrinkage in the production process of the polyimide filmbecause the resulting film tends to have a smaller coefficient of linearexpansion, a higher elastic modulus, and a high birefringence.

<Coefficient of Thermal Diffusivity in Planer Direction of GraphiteFilm>

The coefficient of thermal diffusivity in a planer direction of thegraphite film of the present invention may be not less than 8.0×10⁻⁴m²/s, preferably not less than 8.5×10⁻⁴ m²/s, and more preferably notless than 9.0×10⁻⁴ m²/s. When the coefficient of thermal diffusivity isnot less than 8.0×10⁻⁴ m²/s, the heat can be easily escaped from theheat generating instrument by virtue of high thermal diffusivity,elevation of the temperature of the heat generating instrument can beprevented. To the contrary, when the coefficient of thermal diffusivityis less than 8.0×10⁻⁴ m²/s, the heat can not be escaped from the heatgenerating instrument due to inferior thermal diffusivity, elevation ofthe temperature of the heat generating instrument may not be prevented.

Generally, the flex resistance and thermal diffusivity of the graphitefilm obtained by a polymer degradation method are contradictoryproperties, and the thermal diffusivity is often deteriorated when theflex resistance is sought. However, since the graphite film of thepresent invention has both high thermal diffusivity and flex resistancetogether, it is very advantageous when used in electronic instruments inwhich the amount of heat generation increases while downsizing advances,in recent years.

<Post Planar Compression Step>

In the manufacturing method of the graphite film according to thepresent invention, a step (post planar compression step) of furthercompressing the graphitized source material film, i.e., the graphitefilm via the aforementioned graphitization step into a planer state ispreferably included. This step of post planar compression is animportant step for producing a graphite film that is excellent in flexresistance and coefficient of thermal diffusivity and has superiorflatness without flaw, dint and wrinkle on the surface thereof and alsofor improving the flex resistance, in particular. Such a “post planarcompression step” can be also carried out at a room temperature. In this“post planar compression step”, it is preferred that compression iscarried out into a planar state together with a filmy medium other thanthe graphite film described above. For taking a photograph of theaforementioned SEM image, the operation should be conducted aftercompleting this “post planar compression step”.

In addition, the aforementioned graphite film is preferably compressedinto a planar state while a plurality of pieces of the films are placedto be laminated. Thus, the graphite film itself serves as a buffer,whereby a graphite film can be obtained that is superior in flatnesswithout being flawed on the surface.

Such “post planar compression” may be carried out with veneer pressingor vacuum pressing or the like, and vacuum pressing is particularlypreferred in light of ability to execute compression of the air layerincluded in the graphite film by virtue of the vacuum drawing, inaddition to an advantage of capability of compressing evenly into aplanar state.

More specifically, a method in which a graphite film is compressed usingan apparatus that enables compression into a planar state, such as apressing machine, a hot pressing machine, or a veneer pressing machine,or a method in which a graphite film is sandwiched between plasticplates, ceramic plates, or metal plates and then clamped with a bolt maybe exemplified. By using these methods, the film can be compressedevenly into a planar state, without causing breakage of the graphitelayers, not accompanied by lowering of the coefficient of thermaldiffusivity, and thus a graphite film having a high coefficient ofthermal diffusivity and a high density, and without flaw and wrinkles onthe surface can be obtained. Moreover, in order to provide a moreuniform film, heating may be conducted during the compression.

Moreover, as a method for vacuum pressing, a method in which a vacuumpressing machine, such as a pressing machine, a hot pressing machine ora veneer pressing machine, including a pressing machine to which afunction for vacuum drawing is imparted is used to execute compression,or a method in which a graphite film is sandwiched between plasticplates, ceramic plates, or metal plates and then clamped with a bolt,followed by vacuum drawing the whole, as well as a method in which agraphite film is sandwiched between rubbers as in vacuum rubberpressing, and then the inside is vacuum drawn to allow the pressureinside to be reduced, whereby the film is evenly compressed may beexemplified. According to these methods, in addition to capability ofcompressing evenly into a planar state, compression of the air layerincluded in the graphite film is executed by virtue of the vacuumdrawing, whereby the film can be compressed evenly into a planar state,without causing breakage of the graphite layers, not accompanied bylowering of the coefficient of thermal diffusivity, and thus a graphitefilm having a high coefficient of thermal diffusivity and a highdensity, and without flaw and wrinkles on the surface can be obtained.Additionally, when the vacuum pressing is carried out, vacuum drawing ispreferably conducted prior to the compression. There may be a case inwhich wrinkles are generated when the compression process is carried outfirst; however, when the pressure reduction is conducted beforehand, thegraphite film can be evenly compressed as a whole, whereby a qualitygraphite film can be obtained without generation of wrinkles. Moreover,in order to provide a more uniform film, heating may be conducted duringthe compression also in this method. Since the graphite film isexcellent in thermal diffusivity, the heat can be transferredhomogenously. Accordingly, an advantageous effect of providing a smoothgraphite film being uniform in the face can be achieved.

By thus compressing evenly in the planer direction, stronger compressioncan be carried out as compared with a rolling treatment; therefore, agraphite film that is significantly superior in flex resistance can beobtained as compared with the case of the rolling treatment.

<Filmy Medium>

Illustrative examples of the filmy medium other than the aforementionedgraphite film include graphite films obtained from natural black lead,resin films, metal foils, and the like. Specifically, graphite filmsobtained from natural black lead, shock-absorbing rubber materials, ironplates, Teflon (registered trademark) film, and the like may beexemplified.

The phrase “together with a filmy medium” described above may involveaspects as illustrated in the following: for example, to sandwich in theorder of “a medium other than the graphite film/a piece of theaforementioned graphite film/a medium other than the graphite film/apiece of the aforementioned graphite film/a medium other than thegraphite film/and so forth” etc., to sandwich in the order of “a mediumother than the graphite film/plural pieces of the aforementionedgraphite film/a medium other than the graphite film/plural pieces of theaforementioned graphite film/a medium other than the graphite film/andso forth”, and the like.

<Construction of Graphite Composite Film>

The graphite composite film of the present invention may include aplastic film on at least one face and/or both faces formed via anadhesive material, an adhesive or the like for the purpose of improvingthe flex resistance. In addition, thus combined plastic film may beresponsible for imparting an insulation property to the graphite filmhaving electric conductivity. Examples of the material of the plasticfilm include polyimide, polyethylene terephthalate, epoxy, and the like.These materials are excellent in heat resistance, and satisfactorylong-term reliability can be achieved even though the film is used incombination with a heat-generating component or a heat-dissipatingcomponent.

The plastic film may have a thickness of not greater than 50 μm,preferably not greater than 35 μm, and still more preferably not greaterthan 15 μm. When the thickness is greater than 50 μm, the excellentthermal diffusivity of a graphite film may be impaired when used incombination with the graphite film. Furthermore, the plastic film mayhave a thickness of not less than 10 μm. When the thickness is less than10 μm, sufficient adhesiveness cannot be kept, and long-term reliabilitycan be inferior also when used in combination with a heat-generatingcomponent or a heat-dissipating component.

Further, the thickness of the adhesive material or the adhesive used inlaminating the plastic film may be not greater than 50 μm, preferablynot greater than 35 μm, and still more preferably not greater than 15μm. When the thickness is greater than 50 μm, the excellent thermaldiffusivity of a graphite film may be impaired when used in combinationwith the graphite film. In addition, the adhesive material and theadhesive may be composed of a material such as an acrylic resin, anepoxy resin, a polyimide resin, or the like.

<Application and the Like>

Since the graphite film of the present invention is also excellent inflexibility and electric conductivity, it is particularly suited for usethat takes advantage of these features. The feature the is excellent inheat conductivity of the graphite film is suited for use in whicheffects such as heat transferring ability, heat dissipating ability,heat spreading ability, heat uniformizing ability, accelerating abilityof heat response, quick warming ability, quick cooling ability and thelike may be required. By spreading the heat instantly, sudden change oftemperature can be prevented or moderated, or concentration of localheating can be obviated. Alternatively, to the contrary, it can beapplied to use in which sudden change should be caused, or slight heatchange should be detected. By moderating the heat, the strength andadhesiveness can be ensured even in high-temperature environments. Inaddition, by transferring the heat in an even and accurate manner,improvement of the properties such as high accuracy, high grade and highimage quality are also enabled. In the case of use in manufacturingapparatuses, improvement of productivity is enabled by shortening oftact time, improvement of heat cooling efficiency, improvement of dryingefficiency, speeding up, and shortening of latency time, takingadvantage of the features that enable the heat to transfer rapidly in alarger amount. Additionally, reduction of defects, and enhancement ofkeep warm function are also enabled by virtue of uniformization andquick transfer of the heat. Furthermore, by incorporating into variousinstruments, space-saving, film thinning, weight saving, mechanicalsimplification, and improvement of degree of freedom of settlement areenabled, thereby also capable of saving electric power, and silencing,by eliminating unnecessary components. In addition, since the heat canbe escaped, deterioration of properties does not occur even in heatcycle environment tests and annealing treatments, and solder heatresistant, adhesiveness of adhesion layers, heat resistance, reliabilityand durability can be improved. Also, thermal insulation properties canbe improved, and protection from heat sensitive components are enabled.Consequently, maintenanceless and cost down advantages can be realized,and it would be also possible to enhance safety.

As specific use, the followings may be exemplified as suitableapplications. For example, servers, personal computers for servers,desktop personal computers, word processors, keyboards, electronicinstruments for games, laptop computers, electronic dictionaries, PDAs,mobile phones, portable game instruments, portable electronicinstruments such as portable music players. liquid crystal displays,transmissive liquid crystal display devices, reflective LCD panels,plasma displays, SEDs, LEDs, organic ELs, inorganic ELs, liquid crystalprojectors, rear projectors, liquid crystal panels, backlightapparatuses (for preventing variance, and improving temperaturevariation), TFT substrates, electron emission elements, electron sourcesubstrates and face plates (for weight saving), compounding with displaypanel frames, light emitting elements, optical display instruments andcomponents thereof such as charge-injecting light emitting elements andclocks. Light emitting illumination apparatuses such as lasers,semiconductor lasers, light emitting diodes, fluorescent lights,incandescent lamps, light emitting dots, light emitting element arrays,illumination units, plane light emitting apparatuses, and documentillumination apparatuses. Ink jet printer (ink head) apparatuses andcomponents thereof such as recording heads (heaters, heat insulatingmaterials, thermal storage layers etc.) composed of single or multipleheads for ink jet (ejecting ink utilizing a thermal energy), line head,longer ink heads, solid ink jet apparatuses, heat-dissipating plates forink jet heads, ink cartridges, silicon substrates for ink jet heads, inkjet activating drivers, and heat sources for heating ink jet recordingpaper (halogen lamp heater). Electronic photograph apparatuses and imageforming apparatuses, and components thereof including e.g., as tonercartridges, apparatuses having a laser light source, scanning opticalapparatuses (light rays exit unit, deflection scanning polygonalmirrors, polygonal mirror rotating drive motors, optical componentsleading to photo conductor drams), exposure apparatuses, developmentapparatuses (photo conductor drams, photo receptor members, developmentrollers, development sleeves, cleaning apparatuses), transferapparatuses (transfer rolls, transfer belts, middle transfer beltsetc.), fixing apparatuses (fixing rolls (cores, circumferential members,halogen heaters etc.), surf heaters, electromagnetic induction heaters,ceramic heaters, fixing films, film heating apparatuses, heatingrollers, compression rollers, heating bodies, compression members, beltnips), sheet cooling apparatuses, sheet disposing apparatuses, sheetdischarging apparatuses, sheet processing apparatuses. In fixingapparatuses, the effect of improving heat characteristics exhibited byusing the graphite film is prominent, and image qualities variation inthe width direction, image qualities defects, image qualitiesfluctuation in continuous paper feed, rise and fall time, real timeresponse, high following depending on temperatures, temperaturedifference between paper feed part and unfed part, wrinkle, strength,electric power saving, on-demand heating, high- and low-temperatureoffset, excessive temperature elevation around the heater, and fractureof the heater can be significantly improved. Other recording apparatusessuch as thermal transfer recording apparatuses (ribbons), dot printers,and sublimation printers. Components relating to semiconductors such assemiconductor elements, semiconductor packages, semiconductor sealingcases, semiconductor die bondings, semiconductor chips for drivingliquid crystal display elements, CPUs, MPUs, memories, powertransistors, and power transistor cases. Wiring boards such asprinted-circuit boards, rigid wiring plates, flexible wiring plates,ceramic wiring plates, build-up wiring plates, circuit boards formounting, printed circuit boards for high-density mounting, (tapecarrier packages), TABs, hinge mechanisms, sliding mechanisms, throughholes, resin packagings, sealing materials, multilayer resin moldedproducts, and multilayer boards. Recording apparatuses, recording andreproduction apparatuses and components thereof such as CDs, DVDs (lightpick up, laser generation apparatuses, laser acceptance apparatuses),Blu-ray discs, DRAMs, flash memories, hard disk drives, opticalrecording and reproduction apparatuses, magnetic recording andreproduction apparatuses, magneto optical recording and reproductionapparatuses, information recording media, optical recording disks,magneto optical recording media (translucent substrate, opticalinterference layer, domain wall displacement layer, mid layer, recordinglayer, protective layer, heat-dissipating layer, information track),photo acceptance elements, photo detection elements, light pickupapparatuses, magnetic heads, magnetic heads for magneto opticalrecording, semiconductor laser chips, laser diodes, and laser drivingICs. Image recording apparatuses and components thereof such as digitalcameras, analog cameras, digital single-lens reflex cameras, analogsingle lens reflex cameras, digital cameras, digital video cameras,cameras for integrated VTRs, ICs for integrated VTRs with a camera,lights for video cameras, electronic flare apparatuses, imagingapparatuses, imaging tube cooling apparatuses, imaging apparatuses,imaging elements, CCD elements, lens barrels, image sensors andinformation processing apparatuses using the same, X-ray absorbent corepatterns, X-ray mask structural bodies, X-ray photographic apparatuses,X-ray exposure apparatuses, X-ray plane detectors, X-ray digitalphotographic apparatuses, X-ray area sensor boards, specimen coolingholders for electron microscope, electronic beam drawing apparatuses(electronic guns, electronic guns, electronic beam drawing apparatuses),radial ray detection apparatuses and radial ray imaging systems,scanners, image reading apparatuses, imaging elements for movingpictures and imaging elements for still pictures, and microscopes.Heat-dissipating materials of battery instruments including primaryelectric cells such as alkali cells and manganese cells, secondaryelectric cells such as lithium ion cells, nickel hydrogen andlead-storage electric cells, electric bilayer capacitors, electrolyticcapacitors, battery packs, solar cells, solar cell module installationstructures, photoelectric conversion boards, photoelectromotive forceelement arrays, electric generating elements, and fuel electric cells(electric generating cells, housing exterior portions, fuel tankinterior portions). Electric power supplies and components thereof suchas electric power supplies (rectifier diodes, trans), DC/DC converters,switching electric power supply apparatuses (forward type), electriccurrent leads and superconductive apparatus system. Motor and componentsthereof such as motors, linear motors, plane motors, oscillatory motors,motor coils, circuit units for driving rotational control, motordrivers, inner rotor motors and oscillatory actuators. Deposit filmmanufacturing apparatuses (with constant temperature, for qualitystabilization) and components thereof such as vacuum processingapparatuses, semiconductor manufacturing apparatuses, vapor depositionapparatuses, thin film single-crystalline semiconductor layermanufacturing apparatuses, plasma CVDs, micro wave plasma CVDs,sputtering apparatuses, vacuum pumping apparatuses such as reducedpressure chambers, vacuum pumps and cryo trap-cryo pumps, electrostaticchucks, vacuum chucks, pin chuck type wafer chucks, targets forsputtering, semiconductor exposure apparatuses, lens holding apparatusesand projection exposure apparatuses and photomasks. A variety ofmanufacturing apparatuses and components thereof such as heat treatmentapparatuses by ohmic resistance heating, induction heating, and infraredray heating, dryers, annealing apparatuses, lamination apparatuses,reflowing apparatuses, heating adhesion (contact bonding) apparatuses,injection molding apparatuses (nozzle□heating portions), resin moldingdies, LIM molding, roller molding apparatus modifying gas manufacturing(modifying portions, catalyst portions, heating portions, etc.)stampers, (for filmy, roll, recording media), bonding tools, catalystreaction vessels, chilling machines, coloring apparatus of color filterboards, heat cooling apparatuses of resists, welding instruments, filmsfor magnetic induction heating, dew condensation preventing glasses,residual liquid quality sensing apparatuses and heat exchangeapparatuses. Heat insulation apparatuses such as heat insulatingmaterials, vacuum heat insulating materials and radiation heatinsulating materials. Chassis, housing, outer packaging covers forvarious electronic and electric instruments, and manufacturingapparatuses. Heat-dissipating components such as heat wasters, openings,heat pipes, heat sinks, fins, fans and connectors for heat dissipation.Cooling components such as Peltier elements, electric heat transferelements and water-cooling components. Temperature regulationapparatuses, thermal control apparatuses, temperature detectionapparatuses and components. Heat generator associated components such asthermistors, thermometal cut-out, thermostats, temperature fuses, excessvoltage prevention elements, termoprotectors, ceramic heaters, flexibleheaters, composite products of a heater, a heat-conducting plate and aheat insulating material, and heater connector electrode terminalcomponents. Radiation components having a high emissivity,electromagnetic shielding components such as electromagnetic waveshieldings and electromagnetic wave absorbent cores, composite productswith a metal such as alminum, copper or silicon, composite products witha ceramic such as silicon nitride, boron nitride or alumina.

EXAMPLES

Hereinafter, various Examples of the present invention will be describedalong with several Comparative Examples.

<Graphite Films A and B>

[Production Method of Polyimide Film A]

Pyromellitic dianhydride (1 equivalent) was dissolved in a solutionprepared by dissolving 1 equivalent of 4,4′-oxydianiline in DMF(dimethylformamide) to produce a polyamic acid solution (18.5 wt %).

While cooling the resulting solution, an imidization catalyst containing1 equivalent of acetic anhydride and 1 equivalent of isoquinoline,relative to the carboxylic acid group contained in the polyamic acid,and DMF was added thereto, followed by defoaming. Subsequently, theresulting mixed solution was applied on an aluminum foil such that apredetermined thickness was achieved after drying. The mixed solutionlayer on the aluminum foil was dried using a hot-air oven and afar-infrared heater.

The drying conditions for producing a film having a final thickness of75 μm are shown in the following. The mixed solution layer on thealuminum foil was dried in a hot-air oven at 120° C. for 240 seconds toproduce a self-supporting gel film. The resulting gel film was strippedoff from the aluminum foil and fixed on a frame. The gel film was driedby heating stepwise in a hot-air oven at 120° C. for 30 seconds, at 275°C. for 40 seconds, at 400° C. for 43 seconds, and at 450° C. for 50seconds, and with a far-infrared heater at 460° C. for 23 seconds.

As described in the foregoing, a polyimide film A having a thickness of75 μm (elastic modulus: 3.1 GPa, coefficient of water absorption: 2.5%,birefringence: 0.10, coefficient of linear expansion 3.0×10⁻⁵/° C.) wasproduced.

[Production Method of Polyimide Film B]

A polyimide film B was obtained in a similar manner to the polyimidefilm A except that application was conducted on the aluminum foil togive a final thickness of 50 μm, and the baking time was set to ⅔ foldof that in the case of the final film thickness being 75 μm.

[Production Method of Carbonized Film A]

The polyimide film A having a thickness of 75 μm was sandwiched betweenblack lead plates, and subjected to a carbonization treatment using anelectric furnace, by elevating the temperature to 1,000° C. Thuscarbonized film is designated as carbonized film A.

[Production Method of Carbonized Film B]

A carbonized film B was produced in a similar manner to the carbonizedfilm A except that the polyimide film B having a thickness of 50 μm wasused.

<Measurement of Density of Graphite Film>

The density of the graphite film was calculated by a division processof: dividing the weight (g) of the graphite film by the volume (cm³) ofthe graphite film, i.e., product of length×width×thickness. Thethickness of the graphite film employed was an average value ofmeasurements at any ten positions.

<Measurement of Thickness of Graphite Film>

In a method for measuring the thickness of the graphite film, a 50 mm×50mm film was subjected to measurement at any ten positions using athickness gauge (manufactured by Heidenhain Co., Ltd., HEIDENHAIN-CERTO)in a temperature-controlled room at a room temperature of 25° C., andthe average was derived from the measurements.

<Measurement of Coefficient of Thermal Diffusivity in Planer Directionof Film by Optical Alternating Current Method>

The situation of progress of graphitization is determined by measuring acoefficient of thermal diffusivity in a planer direction of the film,meaning that graphitization is more prominent as the coefficient ofthermal diffusivity is higher. The coefficient of thermal diffusivitywas measured with a graphite film cut into a shape of 4×40 mm specimen,using an apparatus for measuring coefficient of thermal diffusivity(available from ULVAC-RIKO, Inc. “LaserPit”) according to an opticalalternating current method in an atmosphere of 20° C., at 10 Hz.

<MIT Folding Endurance Test of Graphite Film>

A MIT folding endurance test of a graphite film was carried out. Thegraphite film was cut into 1.5×10 cm, and subjected to measurement usinga MIT flex resistant fatigue testing machine model D manufactured byTOYO SEIKI Co., Ltd., at a test load of 100 gf (0.98 N) and a rate of 90times/min, with two series of a curvature radius R of the bending clampbeing 1 mm and 2 mm. Measurement was carried out with respect to abending angle of 135° of two left and right sides.

<Details of SEM Inspection of Graphite Film Surface>

The observation of the surface of a graphite film was carried out usinga scanning electron microscope (SEM) (product name: manufactured byHitachi, model S-4500) at an accelerating voltage of 5 kV. Several kindsof graphite films were cut into pieces of 5 mm×5 mm, and fixed on analminum stage having a diameter of 15 mm with an electrically conductivetape. The stage is regulated and set to give a height of 36 mm. Theaccelerating voltage was regulated to be 5 eV, and observation with ahigh magnification mode at 400× was carried out. The working distancewas regulated to be 8 mm, and then the brightness, the contrast and thefocus were adjusted. Thus, photographing was carried out such that thewrinkle of the graphite film surface can be observed. The image wascaptured at 640×480.

<Image Processing of SEM Inspection Image on Graphite Film Surface>

Image processing of the image obtained by SEM inspection of the graphitefilm surface was carried out using a general-purpose image processingsoft (product name: NANO HUNTER NS2K-PRO/LT) available from NANO SystemCorporation. With respect to details of the image processing, the SEMimage was first incorporated into the image processing program, and thedensity measurement was carried out. The maximum value (maximum: 255)and the minimum value (0) measured with the density measurement wereconfirmed, and the threshold of the binarization was determinedaccording to the following formula.Threshold=(maximum value−minimum value)×0.62  (1)

Next, the SEM image was binarized with the threshold determinedaccording to the above formula. In the binarization processing, a regionrevealing brighter than a certain threshold is whitened, while a regionrevealing darker than a certain threshold is blackened.

Subsequently, the binarized image was thinned. In the thinningprocessing, the whitened portion in the binarized image is converted ina line width of 1.

The area of the white region of the image derived by the processing asdescribed above was measured.

<Production of Graphite Composite Film>

Composite with PET Tape

A PET film was formed on both faces of a graphite film via an acrylicadhesive material. Specifically, as shown in FIG. 12, a 15×100 mmgraphite film and a 17×100 mm PET tape (manufactured by TeraokaSeisakusho Co. Ltd., 631S#12) were pasted with a laminating machine. Inthis step, pasting was carried out with 1 mm of the PET tape extendingout from both two ends of the graphite film.

Composite with Virtual Flexible Print Wiring Plate

A graphite film was combined with a flexible print wiring plate via anadhesive material. Specifically, a 15×100 mm graphite film was combinedwith a 17×100 mm PET tape (manufactured by Teraoka Seisakusho Co., Ltd.,631S#12) using a 17×100 mm adhesive material (manufactured by TeraokaSeisakusho Co., Ltd., 707) so as to enclose the same, as shown in FIG.13, and was laminated with a virtual model of flexible board constitutedwith a 12.5 μm polyimide film (manufactured by Kaneka Corporation,Apical AH) and a 35 μm copper foil using a laminating machine.

<MIT Folding Endurance Test of Composite>

The composite of the PET tape and graphite, and the composite with theflexible print wiring plate were evaluated using a MIT folding endurancetester. A MIT flex resistant fatigue testing machine model Dmanufactured by TOYO SEIKI Co., Ltd. was used for measurement with atest load of 100 gf (0.98 N), at a rate of 90 times/min, and a curvatureradius R of the bending clamp being 1 mm, with a bending angle of 90° ofleft and right sides. The bent portions after flex of 50, 100, 1,000,5,000, 10,000, 50,000, 100,000, and 200,000 times were visuallyobserved, and the numbers of the bent portion kept without alterationare shown in Table. It should be noted that states in which exfoliationfrom between the graphite layers was found, in which the PET tape wasloosen, and the like were decided as “alteration found”, while thestates in which seams (including color change) were found correspondingto the flex at the bent portion were decided as “alteration not found”.

<Evaluation of Cooling Performance after Seam Folding>

Deterioration of thermal diffusivity before and after seam folding ofthe graphite was evaluated.

With respect to the seam folding, specifically, when R at the bentportion is 1, a 20×100 mm graphite film was produced, and a portion fromthe top to 20 mm away therefrom was folded, and a weight of 500 g wasplaces. In addition, the film was further folded from the same portionin a opposing direction, and a weight of 500 g was similarly placed,which operation was counted as suggesting number of times of seamfolding being once. When R is 0.5, a wire having a diameter of 0.5 mmwas inserted into the portion, and similar folding operation was carriedout.

For evaluation of the thermal diffusivity, a 1 cm square ceramic heater(manufactured by SAKAGUCHI E.H VOC Corp.) was connected to the graphitefilm before or after seam folding as a heat generator, as shown in FIG.14. For connection, a high thermal diffusivity silicone rubber sheet(manufactured by GELTEC, 6.5 W/mK) for use in contacting a heat sinkwith CPU was employed. Additionally, the room temperature was regulatedto 22.5±0.5° C., and the measurement was carried out while covering themeasurement region by foamed polystyrene such that the cooling effect byvirtue of convection (wind) could kept constant. The wattage of theelectric power supply was kept at 1 W to carry out the measurement. Whena steady state (heater temperature being no higher than ±1° C.) wasobserved, the heater temperature was measured. The measurement with theheater was carried out using a thermometer for reading the infrared raygenerated from the heat generator. In practice, Thermotracer TH9100MV/WV(manufactured by NEC San-ei Instruments, Ltd.) was used to measure theheater temperature. Higher temperature of the heater would indicatedeterioration of the cooling performance of the graphite.

The thickness, the percentage of the white region after the surface SEMimage processing, the number of times in the MIT test, the coefficientof thermal diffusivity, density, and the like of the of the graphitefilm used in Examples and Comparative Examples determined by the presentinventors or applicants of the present invention are summarized inTable 1. It should be noted that the graphite production method ofReference Examples shown in Table 1 were estimated according to knownbibliographies.

TABLE 1 Graphiti- Number of Ratio of White zation Pressure LaminatedThickness Region After Source Max During Graphite After Surface SEMImage Graphite Material Method for Temp. Graphitizing film CompressionProcessing Film Film Manufacturing (° C.) (g/cm²) (pieces) (μm) (%)Example 1 Graphite film Polyimid A Polymer degradation method 3000 30100 40 2.5 1 (electric heating) Example 2 Graphite film Polyimid BPolymer degradation method 3000 30 100 25 2.6 2 (electric heating)Example 3 Graphite film Polyimid A Polymer degradation method 2900 30100 41 3.2 3 (electric heating) Example 4 Graphite film Polyimid APolymer degradation method 2800 30 100 42 3.4 4 (electric heating)Example 5 Graphite film Polyimid A Polymer degradation method 3000 30 2039 2.3 5 (electric heating) Example 6 Graphite film Polyimid A Polymerdegradation method 3000 30 100 49 7.9 6 (electric heating) ComparativeGraphite film Polyimid A Polymer degradation method 2600 30 100 46 0.9Example 1 7 (electric heating) Comparative Graphite film Polyimid APolymer degradation method 3000 30 1 37 0.4 Example 2 8 (electricheating) Comparative Graphite film Polyimid A Polymer degradation method3000 30 5 38 0.8 Example 3 9 (electric heating) Comparative Graphitefilm Polyimid A Polymer degradation method 3000 120 100 37 0.9 Example 410 (electric heating) Comparative Graphite film Polyimid A Polymerdegradation method 3000 3 100 43 8.6 Example 5 11 (electric heating)Reference PGS(70 μm KAPTON75 Polymer degradation method — — — 70 8.9Example 1 product) Reference PGS(100 μm KAPTON75 Polymer degradationmethod — — — 100 9.4 Example 2 product) Evaluation of heat conductivityafter Flex resistance properties seam folding, 10 Composite Film timesDensity Coefficient Composite with Difference of heater After of ThermalSingle Film PET Sandwich Flexible Plate Temp. between before compressionDiffusivity R = 1 R = 2 R = 1 and after seam folding (g/cm³) (×10⁻⁴m²/s)135 deg. 135 deg. 90 deg. R = 0 R = 0.5 Example 1 1.75 9.1 98342 times104352 times 100000 times or 50000 times or 0.2° C. 0.0° C. more moreExample 2 1.75 9.2 78291 times 92923 times 100000 times or 50000 timesor 0.3° C. 0.2° C. more more Example 3 1.65 8.8 40030 times 50899 times50000 times or 10000 times or 0.8° C. 0.1° C. more more Example 4 1.608.3 24533 times 43349 times 50000 times or 10000 times or 1.5° C. 0.2°C. more more Example 5 1.80 9.3 14899 times 12398 times 50000 times or5000 times or 2.0° C. 0.1° C. more more Example 6 1.49 7.2 10006 times11239 times 50000 times or 5000 times or 2.1° C. 0.1° C. more moreComparative 2.00 3.2 8 times 9 times 50 

 times or 50 times or Unmeasurable Unmeasurable Example 1 less lessComparative 2.10 10.1 5 times 8 times 50 

 times or 50 times or Unmeasurable Unmeasurable Example 2 less lessComparative 2.05 9.8 33 times 17 times 50 

 times or 50 

 times Unmeasurable Unmeasurable Example 3 less or less Comparative 2.0010.3 278 times 346 times 1000 times or 1000 times or UnmeasurableUnmeasurable Example 4 more more Comparative 1.45 8.2 3420 times 4239times 10000 times or 1000 times or Unmeasurable Unmeasurable Example 5more more Reference 1.20 7.2 1176 times 2040 times 10000 times or 5000times or 2.3° C. 0.1° C. Example 1 more more Reference 0.90 7.0 342times 453 times 10000 times or 5000 times or 3.4° C. 1.1° C. Example 2more more (*Source materials of Reference Examples were extructed fromDocuments.)

Example 1

A source material film laminate in which 100 pieces of carbonized filmsA (400 cm² (length: 200 mm×width: 200 mm) obtained by a carbonizingtreatment were laminated was sandwiched with platy smooth graphiteshaving a length of 270 mm×a width of 270 mm×a thickness of 3 mm fromboth top and bottom, and held in a black lead vessel having a length 300mm×a width of 300 mm×a thickness of 60 mm (vessel A) as shown in FIG.15. A platy graphite as a weight was placed on the laminate so as toallow the pressure of 30 g/cm² to be applied on the film, and agraphitization treatment was permitted by elevating the temperature to3,000° C. using a graphitization furnace. The vessel A was held directlyin an electrically accessible vessel B as schematically shown in FIG.16, and around the outer periphery of the vessel A was covered withcarbon powders (filling carbon powders with vessel A and vessel B). Asshown in FIG. 16, electric heating was conducted by applying a voltagein the state in which carbon powders were covered around the outerperiphery of the vessel B until the temperature of the vessel A reaches3,000° C. After cooling to a room temperature, the graphite after theheat treatment was compressed by a veneer press method to obtain agraphite film 1. Using the graphite film 1, various physical propertiesshown in Table 1 of the simplicial graphite film and the composite weredetermined.

Example 2

A graphite film 2 was produced in a similar manner to the graphite 1except that the carbonized film B was used as a source material. Usingthe graphite film 2, various physical properties shown in Table 1 of thesimplicial graphite film and the composite were determined.

Example 3

A graphite film 3 was produced in a similar manner to the graphite 1except that the graphitization treatment was carried out by elevatingthe temperature to 2,900° C. with a graphitization furnace. Using thegraphite film 3, various physical properties shown in Table 1 of thesimplicial graphite film and the composite were determined.

Example 4

A graphite film 4 was produced in a similar manner to the graphite 1except that the graphitization treatment was carried out by elevatingthe temperature to 2,800° C. with a graphitization furnace. Using thegraphite film 4, various physical properties shown in Table 1 of thesimplicial graphite film and the composite were determined.

Example 5

A graphite film 5 was produced in a similar manner to the graphite 1except that twenty pieces of the carbonized film A obtained by thecarbonization treatment were laminated. Using the graphite film 5,various physical properties shown in Table 1 of the simplicial graphitefilm and the composite were determined.

Example 6

A graphite film 6 was produced in a similar manner to the graphite 1except that a heater of ohmic resistance heating system was used, andthe vessel A was heated to 3,000° C. in an argon gas stream. Using thegraphite film 6, various physical properties shown in Table 1 of thesimplicial graphite film and the composite were determined.

Comparative Example 1

A graphite film 7 was produced in a similar manner to the graphite 1except that the graphitization treatment was carried out by elevatingthe temperature to 2,600° C. with a graphitization furnace. Using thegraphite film 7, various physical properties shown in Table 1 of thesimplicial graphite film and the composite were determined.

Comparative Example 2

A graphite film 8 was produced in a similar manner to the graphite 1except that the carbonized film A obtained by the carbonizationtreatment was not laminated but a single piece was set. Using thegraphite film 8, various physical properties shown in Table 1 of thesimplicial graphite film and the composite were determined.

Comparative Example 3

A graphite film 9 was produced in a similar manner to the graphite 1except that five pieces of the carbonized film A obtained by thecarbonization treatment were laminated. Using the graphite film 9,various physical properties shown in Table 1 of the simplicial graphitefilm and the composite were determined.

Comparative Example 4

A graphite film 10 was produced in a similar manner to the graphite 1except that a platy graphite as a weight was placed on the laminate soas to allow the pressure of 120 g/cm² to be applied on the sourcematerial film laminate. Using the graphite film 10, various physicalproperties shown in Table 1 of the simplicial graphite film and thecomposite were determined.

Comparative Example 5

A graphite film 11 was produced in a similar manner to the graphite 1except that a platy graphite as a weight was placed on the laminate soas to allow the pressure of 3 g/cm² to be applied on the source materialfilm laminate. Using the graphite film 11, various physical propertiesshown in Table 1 of the simplicial graphite film and the composite weredetermined.

Reference Example 1

Using a generally available PGS graphite film “EYGS182310” manufacturedby Matsushita Electric Industrial Co., Ltd., various physical propertiesshown in Table 1 of the simplicial graphite film and the composite weredetermined.

Reference Example 2

Using a generally available PGS graphite film “EYGS182310” manufacturedby Matsushita Electric Industrial Co., Ltd., various physical propertiesshown in Table 1 of the simplicial graphite film and the composite weredetermined.

MIT Folding Endurance Test of Simplicial Graphite Film>

<R=2, 135°>

The number of foldings according to the MIT test on Examples 1 to 6 was10,000 times or more, which indicated a very higher flex resistance ascompared with Comparative Examples 1 to 7 and Reference Examples 1 to 2.

Particularly, a significantly higher flex resistance with the number of100,000 times or more was indicated in Example 1, and this results fromuniform heating of the source material film since the graphitization wascarried out with an electric heating method.

In addition, since the temperature was elevated to 3,000° C., thegraphite layers grown in the planer direction, whereby the mechanicalproperties in the planer direction were improved.

Furthermore, as a result of the number of the laminated pieces of thesource material film being 100, excessive heating in an even mannercould be prevented, and generation of the gas to yield the foaming statecould be retarded, to enable suppression of invasion of impurities.

In addition, since the balance between the applied load and the numberof the laminated pieces of the source material film was appropriate, theextent of foaming was suppressed, and the growth of the graphite layerin the planer direction was accelerated.

Moreover, the polymer film used as the source material having amolecular orientation suited for obtaining a graphite that was excellentin the flex resistance would be one factor for such events.

Accordingly, due to a synergistic effect of such several factors thatimprove the flex resistance, a graphite film that is extremely excellentin the flex resistance could be obtained in Example 1 compared withExamples 3 to 6, Comparative Examples 1 to 5, and Reference Examples 1to 2.

In Example 2, a graphite film having a thickness of 25 μm was obtainedsince a source material film having a smaller thickness than Example 1was used. Thus, the flex resistance of the film was inferior to the filmof Example 1 although the extent was just a small.

The flex resistance of the graphite films of Examples 3 to 4 wasinferior as compared with that of Example 1 on the ground that thetemperature was not elevated just to 2,900° C. and 2,800° C. in Examples3 and 4, whereby growth of the graphite layers in the planer directionwas insufficient as compared with Example 1. However, although Examples3 to 4 exhibited somewhat inferior results compared with Examples 1 to2, graphite films that are excellent in the flex resistance as comparedwith Comparative Examples 1 to 5 and Reference Examples 1 to 2 wereobtained since other baking conditions were appropriately set.

The flex resistance of the graphite film of Example 5 was inferior ascompared with that of Example 1 on the ground that the number of thelaminated pieces of the source material film was small, whereby the filmwas heated in an even manner excessively, gas generation could not beretarded, and invasion of impurities could not be suppressed, ascompared with Example 1. However, although Example 5 exhibited somewhatinferior results compared with Example 1, a graphite film that isexcellent in the flex resistance as compared with Comparative Examples 1to 5 and Reference Examples 1 to 2 was obtained since other bakingconditions were appropriately set.

The flex resistance of the graphite film of Example 6 was inferior ascompared with that of Example 1 on the ground that the specimen was notevenly heated in Example 6 due to the atmospheric heating employed.However, although Example 6 exhibited somewhat inferior results comparedwith Example 1, a graphite film that is excellent in the flex resistanceas compared with Comparative Examples 1 to 5 and Reference Examples 1 to2 was obtained since other baking conditions were appropriately set.

On the other hand, Comparative Examples 1 to 6 exhibited significantlyinferior flex resistance as compared with Examples 1 to 6.

The flex resistance was inferior in Comparative Example 1 as comparedwith Examples on the ground that graphitization did not proceed enoughdue to the maximum temperature as low as 2,600° C., and that sufficientfoaming state could not be achieved since the temperature was notelevated to the temperature that allows the internal gas to begenerated.

The flex resistance was inferior in Comparative Examples 2 and 3 ascompared with Examples on the ground that the number of the laminatedpieces of the source material films in graphitization was small. Thiswould result from small number of the laminated pieces, whereby the filmwas heated in an even manner excessively, the gas generation could notbe retarded, and invasion of impurities could not be suppressed.

The flex resistance was inferior in Comparative Example 4 as comparedwith Examples on the ground that the load placed in graphitization wastoo heavy, whereby foaming of the film in the thickness direction wassuppressed, and the graphite layers were excessively grown in the planerdirection.

The flex resistance was inferior in Comparative Example 5 as comparedwith Examples on the ground that the load placed in graphitization wastoo light, and thus, the growth of the graphite layers in the planerdirection was insufficient, and the extent of foaming was too large.

Furthermore, significantly inferior flex resistance was exhibited inReference Examples 1 to 2 as compared with Examples 1 to 4.

<R=1, 135°>

When the bending radius R was 1, a similar tendency was observed to thecase in which R is 2, although the number of times of the flex wassmaller. Thus, description of details is herein omitted.

<Percentage of White Region after Surface SEM Image Processing>

Images obtained by image processing of surface SEM photographs ofExamples 1 to 6, Comparative Examples 1 to 5, and Reference Examples 1to 2 are shown in FIG. 17 to FIG. 29.

The percentage of the white region after the surface SEM imageprocessing of Examples 1 to 6 was in the range of not less than 2.3% andnot greater than 7.9%, and thus foaming states found in graphite filmsthat are excellent in the flex resistance could be ascertained.

In more detail, Examples 1 and 2 exhibited the percentage of 2.5% and2.3%, respectively, suggesting particularly excellent flex resistance byvirtue of very appropriate foaming states. This results from uniformheating of the source material film since the graphitization was carriedout with an electric heating method.

In addition, since the temperature was elevated to 3,000° C., thegraphite layers grown in the planer direction, whereby the mechanicalproperties in the planer direction were improved.

Furthermore, as a result of the number of the laminated pieces of thesource material film being 100, excessive heating in an even mannercould be prevented, and generation of the gas could be retarded, toenable suppression of invasion of impurities, whereby the graphite filmsthat exhibit excellent flex resistance could be obtained.

In addition, as a result of the balance between the applied load and thenumber of the laminated pieces of the source material film beingappropriate, the extent of foaming was suppressed, and the growth of thegraphite layer in the planer direction was accelerated, whereby graphitefilms that are excellent in the flex resistance could be obtained.

Moreover, the polymer film used as the source material having amolecular orientation suited for obtaining a graphite that was excellentin the flex resistance would be one factor for such events.

Accordingly, due to a synergistic effect of such several factors thatimprove the flex resistance, a graphite film that has extremelyappropriate foaming state could be obtained in Examples 1 to 2 comparedwith Examples 3 to 6, Comparative Examples 1 to 5, and ReferenceExamples 1 to 2.

The percentage of the white region after the surface SEM imageprocessing was greater to some extent in Examples 3 and 4 as comparedwith Examples 1 and 2, on the ground that growth of the graphite layersin the planer direction was insufficient as a result of the maximumtemperature being lower as compared with Examples 1 and 2. On the sameground, the number of times of the MIT test results was smaller ascompared with Examples 1 and 2.

In Example 5, the percentage of the white region after the surface SEMimage processing was smaller to some extent as compared with Examples 1and 2. This would result from small number of the laminated pieces ofthe source material film, leading to a small extent of foaming, as wellas a large domain, since the film was heated in an even mannerexcessively, gas generation could not be retarded, and invasion ofimpurities could not be suppressed, as compared with Examples 1 to 2. Onthe same ground, the number of times of the MIT test results was smalleras compared with Examples 1 and 2.

In Example 6, the percentage of the white region after the surface SEMimage processing was larger as compared with Examples 1 and 2. Thiswould result from the atmospheric heating employed, leading to a smallextent of foaming, as well as a large domain, since the specimen was notheated evenly. On the same ground, the number of times of the MIT testresults was smaller as compared with Examples 1 and 2.

To the contrary, in Comparative Examples 1 to 4, the percentage of thewhite region after the surface SEM image processing was smaller ascompared with Examples 1 to 6. Additionally, in Comparative Example 5,the percentage of the white region after the surface SEM imageprocessing was larger as compared with Examples 1 to 6.

In more detail, the percentage of the white region after the surface SEMimage processing was smaller in Comparative Example 1 as compared withExamples on the ground that graphitization did not proceed enough due tothe maximum temperature as low as 2,600° C., and that foaming failedsince the temperature was not elevated to the temperature that allowsthe internal gas to be generated, and thus, a soft graphite film couldnot be obtained consequently.

The percentage of the white region after the surface SEM imageprocessing was smaller in Comparative Examples 2 and 3 as compared withExamples on the ground that the number of the laminated pieces of thesource material film in graphitization was small. This would result fromsmall number of the laminated pieces, whereby the film was heated in aneven manner excessively, the gas generation could not be retarded, andinvasion of impurities could not be suppressed. As a result, a graphitefilm having a small extent of foaming, a too large domain, and beinginferior in the flex resistance was obtained.

The percentage of the white region after the surface SEM imageprocessing was smaller in Comparative Example 4 as compared withExamples on the ground that the load placed in graphitization was tooheavy, whereby foaming of the film in the thickness direction wassuppressed.

The percentage of the white region after the surface SEM imageprocessing was larger in Comparative Example 5 as compared with Exampleson the ground that the load placed in graphitization was too light, andthus, the growth of the graphite layers in the planer direction wasinsufficient, and the extent of foaming was too large.

Furthermore, the percentage of the white region after the surface SEMimage processing was significantly greater in Reference Examples 1 to 2as compared with Examples 1 to 5. The graphite film had a larger extentof foaming, and a smaller domain as compared with Examples 1 to 5, andtherefore, the flex resistance was inferior to those in Examples 1 to 5.

<Measurement of Coefficient of Thermal Diffusivity>

The coefficient of thermal diffusivity was as high as not less than8.3×10⁻⁴ m²/s, and extremely excellent flex resistance was exhibited inExamples 1 to 5. This would result from the extent of foaming of thegraphite film controlled by the balance of the number of the laminatedpieces and the weight, and uniform graphitization at a high temperatureachieved with an electrification excessive heating method.

Since the graphite film of Example 6 was obtained by an atmosphericheating method, the coefficient of thermal diffusivity was lower to someextent as compared with Examples 1 to 5 due to failure in heating in aneven manner.

To the contrary, the coefficient of thermal diffusivity was as low as3.2×10⁻⁴ m²/s in Comparative Example 1 since the temperature waselevated to 2,600° C., due to insufficient progress of graphitization.

The coefficient of thermal diffusivity was prominent being not less than9.8×10⁻⁴ m²/s in Comparative Examples 2 and 3, and this would resultfrom the number of the laminated pieces of the source material filmbeing small in graphitization. When the number of the laminated piecesis small, the film can be heated evenly, and invasion of impuritiescannot be suppressed, thereby yielding a graphite film in which thegraphite is extremely highly grown in the planer direction.

The coefficient of thermal diffusivity was prominent being not less than10.3×10⁻⁴ m²/s in Comparative Example 4, and this would result fromexcessively high load placed in graphitization, whereby a film includinggraphite layers highly oriented in the planer direction was formed. Thegraphite films of Comparative Examples 2, 3 and 4 were hard and inferiorin the flex resistance due to too small extent of foaming, with domainsgrown to larger, although the coefficient of thermal diffusivity wasvery high. The coefficient of thermal diffusivity of the graphite filmof Reference Examples 1 to 2 were smaller as compared with Examples 1 to5.

<MIT Folding Endurance Test of Composite>

MIT tests were performed on the composite with the PET tape, and thecomposite with the flexible print wiring plate. As a result, thecomposite with the PET tape exhibited more superior flex resistance thanthe composite with the flexible print wiring plate. This is presumed toresult from the composite with the flexible print wiring plate having agreater thickness of the composite, being more rigid, and being morelikely to be deteriorated due to different balance of the top and bottomfaces of the graphite. However, since the composite with the PET tapeand the composite with the flexible print wiring plate indicated asimilar tendency, only the composite with the PET tape will be discussedin the following.

No alteration of the appearance was found even after flex of 50,000times in Examples 1 to 5, which revealed superior flex resistance ascompared with Comparative Examples 1 to 5, and Reference Examples 1 to2. This would result from extremely excellent flex resistance of thesimplicial graphite. To the contrary, according to Comparative Examples1 to 5 and Reference Examples 1 to 2 in which the simplicial graphitefilm was inferior in the flex resistance, very inferior flex resistancewas exhibited even though the film was reinforced with a support of thePET tape.

<Deterioration of Thermal Diffusivity Before and after Seam Folding>

With respect to deterioration of the thermal diffusivity after the seamfolding, any deterioration of thermal diffusivity at 1° C. or higher wasnot observed when R is 0.5 in Examples 1 to 5 and Reference Example 1.To the contrary, the specimen was broken from the bent portion afterseam folding of 10 times in Comparative Examples 1 to 5; therefore,evaluation of the thermal diffusivity could not be performed. Inaddition, deterioration of thermal diffusivity at 1.1° C. was confirmedin Reference Example 2.

Before and after seam folding under a stringent condition of R being0.0, deterioration of the thermal diffusivity at 1° C. or higher wasobserved in Examples 4 to 6 and Reference Examples 1 to 2. In addition,the specimen was broken from the bent portion after seam folding of 10times in Comparative Examples 1 to 5; therefore, evaluation of thethermal diffusivity could not be performed. To the contrary,deterioration of the thermal diffusivity was not found even under astringent condition of R being 0.0 in Examples 1 to 3.

As in the foregoing, it is proven that the graphite film of the presentinvention has an excellent flex resistance which can withstandapplication to bent portions, along with an excellent thermaldiffusivity that enables a heat to be quickly diffused from a site ofheat generation. In addition, it is also proven that the graphitecomposite film produced using the graphite film of the presentinvention, exhibits an excellent flex resistance.

The invention claimed is:
 1. A graphite film obtained by subjecting asource material film consisting of a polymer film and/or a carbonizedpolymer film to a heat treatment at a temperature of 2,000° C. orhigher, wherein said graphite film has the area of a white regionaccounting for not less than 1.0% and not greater than 8.5%, the area ofthe white region being determined by (a) obtaining an SEM image of saidgraphite film surface captured at 640×480 pixels, a magnification of400×, and an accelerating voltage of 5 kV such that a wrinkle of thegraphite film surface is observed, (b) carrying out a densitymeasurement of the SEM image to obtain an SEM image having 256gradations of from a minimum value 0 to maximum value 255 in order todetermine an amount of the boundary lines on the surface of the graphitefilm, (c) binarizing the SEM image in step (b) to black and white with ablack/white threshold defined by the following formula:Threshold=(maximum value−minimum value)×0.62, wherein a region brighterthan the threshold is white and a region darker than the threshold isblack, (d) thinning of the white region of the binarized SEM image intoa line width of 1, and (e) measuring the area of a white region of thebinarized SEM image.
 2. The graphite film according to claim 1, whereinthe graphite film exhibits a number of reciprocal folding being 10,000times or more as measured according to a folding endurance test with arectangular strip test piece having a width of 15 mm until the testpiece breaks under conditions of: a curvature radius R of a bendingclamp being 2 mm; a left-and-right bending angle being 135 degrees; abending rate being 90 times/min; and a load being 0.98 N.
 3. Thegraphite film according to claim 2, said graphite film exhibiting thenumber of reciprocal folding being 10,000 times or more as measuredaccording to a folding endurance test with a rectangular strip testpiece having a width of 15 mm until the test piece breaks underconditions of: a curvature radius R of a bending clamp being 1 mm; aleft-and-right bending angle being 135 degrees; a bending rate being 90times/min; and a load being 0.98 N.
 4. The graphite film according toclaim 2, wherein the graphite film has a coefficient of thermaldiffusivity in a planar direction of not less than 8.0×10⁻⁴ m²/s.
 5. Thegraphite film according to claim 1, said graphite film exhibiting anumber of reciprocal folding being 10,000 times or more as measuredaccording to a folding endurance test with a rectangular strip testpiece having a width of 15 mm until the test piece breaks underconditions of: a curvature radius R of a bending clamp being 1 mm; aleft-and-right bending angle being 135 degrees; a bending rate being 90times/min; and a load being 0.98 N.
 6. The graphite film according toclaim 5, wherein the graphite film has a coefficient of thermaldiffusivity in a planar direction of not less than 8.0×10⁻⁴ m²/s.
 7. Thegraphite film according to claim 1, wherein the graphite film has acoefficient of thermal diffusivity in a planar direction of not lessthan 8.0×10⁻⁴ m²/s.
 8. A graphite composite film comprising a plasticfilm formed on a part of any one of the graphite film according to claim1 via an adhesive material or adhesive.
 9. A graphite composite filmcomprising a plastic film formed on a part of any one of the graphitefilm according to claim 2 via an adhesive material or adhesive.
 10. Agraphite composite film comprising a plastic film formed on a part ofany one of the graphite film according to claim 5 via an adhesivematerial or adhesive.
 11. A graphite composite film comprising a plasticfilm formed on a part of any one of the graphite film according to claim3 via an adhesive material or adhesive.
 12. A graphite film having anarea of a white region accounting for not less than 1.0% and not greaterthan 8.5%, the area of the white region being defined by (a) obtainingan SEM image of said graphite film surface captured at 640×480 pixels, amagnification of 400×, and an accelerating voltage of 5 kV such that awrinkle of the graphite film surface is observed, (b) carrying out adensity measurement of the SEM image to obtain an SEM image having 256gradations of from a minimum value 0 to maximum value 255 in order todetermine an amount of the boundary lines on the surface of the graphitefilm, (c) binarizing the SEM image in step (b) to black and white with ablack/white threshold defined by the following formula:Threshold=(maximum value−minimum value)×0.62, wherein a region brighterthan the threshold is white and a region darker than the threshold isblack, (d) thinning of the white region of the binarized SEM image intoa line width of 1, and (e) measuring the area of a white region of thebinarized SEM image.
 13. The graphite film according to claim 12,wherein the graphite film exhibits a number of reciprocal folding being10,000 times or more as measured according to a folding endurance testwith a rectangular strip test piece having a width of 15 mm until thetest piece breaks under conditions of: a curvature radius R of a bendingclamp being 2 mm; a left-and-right bending angle being 135 degrees; abending rate being 90 times/min; and a load being 0.98 N.
 14. Thegraphite film according to claim 13, wherein the graphite film has acoefficient of thermal diffusivity in a planar direction of not lessthan 8.0×10⁻⁴ m²/s.
 15. The graphite film according to claim 12, saidgraphite film exhibiting a number of reciprocal folding being 10,000times or more as measured according to a folding endurance test with arectangular strip test piece having a width of 15 mm until the testpiece under conditions of: a curvature radius R of a bending clamp being1 mm; a left-and-right bending angle being 135 degrees; a bending ratebeing 90 times/min; and a load being 0.98 N.
 16. The graphite filmaccording to claim 15, wherein the graphite film has a coefficient ofthermal diffusivity in a planar direction of not less than 8.0×10⁻⁴m²/s.
 17. The graphite film according to claim 12, said graphite filmexhibiting a number of reciprocal folding being 10,000 times or more asmeasured according to a folding endurance test with a rectangular striptest piece having a width of 15 mm until the test piece under conditionsof: a curvature radius R of a bending clamp being 1 mm; a left-and-rightbending angle being 135 degrees; a bending rate being 90 times/min; anda load being 0.98 N.
 18. The graphite film according to claim 12,wherein the graphite film has a coefficient of thermal diffusivity in aplanar direction of not less than 8.0×10⁻⁴ m²/s.
 19. A graphitecomposite film comprising a plastic film formed on a part of any one ofthe graphite film according to claim 12 via an adhesive material oradhesive.
 20. A graphite composite film comprising a plastic film formedon a part of any one of the graphite film according to claim 13 via anadhesive material or adhesive.
 21. A graphite composite film comprisinga plastic film formed on a part of any one of the graphite filmaccording to claim 15 via an adhesive material or adhesive.